1. Yes it was somewhat normal for soldiers of that age to enter into the army and stay retained. In this period (the late 1800s) the US Army was much much smaller than it is today. There were less than 40,000 total servicemen across all of the branches.

This did not stray away from allowing older men to enlist. While physical capabilities were still very important, there weren’t really “fitness tests” like the modern military, meaning there wasn’t really a minimum requirement to stay in. Also, there weren’t really height and weight requirements, or any of the other factors modern militaries used to determine whether or not someone was eligible for service in the military. There were fitness tests used for cadets at West Point, but for the average private, not really. It’s important to mention as well. That most men were physically capable enough as well to serve in this period.

It was also very common for older men to enlist, and while you see it most during times of war with men as old as their 40s enlisting as privates.

2. Yes it was extremely common, and most men most likely never reached above private if they enlisted. A troop usually only consisted of a maximum of 95 men (although it was reduced in the early 1890s until the Spanish-American war). And they were allotted 10 total Non-commissioned officers per troop. With 78 other privates in the troop it would mean there was a lot of competition if there was ever an opening. Men were also not promoted like the current system, and many times they were voted on by the men of the company (this was usually in the case of volunteer units though).

Units were also organized on a system of billets. Each unit would be allotted a certain number of billets for each position (I.e. 1 captain, 1 first lieutenant, 1 second lieutenant, etc) meaning that men were appointed to serve in a billet granting them a rank opposed to them carrying that rank to any unit. So if you switched from let’s the 4th Cavalry Regiment to the 7th Cavalry Regiment if you were a first sergeant in the 4th cavalry, unless there was an open first sergeant billet in the 7th cavalry that you were appointed too, you would end up as just a simple private once again.

3. At this time the US Army in California was at the tail end of the Indian Wars. While there countinued to be some conflict with Native American tribes into the 1900s, the period known as the Indian wars in US Military History ends in the 1890s as most significant resistance had been defeated or treaties had been signed with native tribes.

Your great-grandfather probably served at various military installations protecting the frontier and settlers. Without exact units and dates of when he was with them it’s hard to tell exactly what he did. However if he served from 1887-1892 in the 4th Cavalry it looks like he would have missed their final participation in the Indian Wars. In 1890 they were positioned in California, Washington, Oregon, and Idaho. He would pretty much have just been occupying a fort, protecting local settlers and frontiersman, and if the federal government needed them to do anything they would do it.

4. That method of enlistment was not unusual. For enlisted soldiers. Most soldiers would have enlisted with other volunteer units from their communities and served in units all together. However in the case for your great-grandfather he most likely directly joined each unit he would served with and received his training through drilling with said unit. Like I mentioned earlier units were determined on billets and so if you went to any military post attempting to enlist they would let you know whether or not they needed anybody. And to go back to an earlier point if they needed you and you could do it age would not have mattered either.

Sources:

https://www.alternatewars.com/BBOW/Stats/US_Mil_Manpower_1789-1997.htm

http://www.history-magazine.com/cavalry.html

https://history.army.mil/html/forcestruc/usa-1890.html

(Myself)

P.s. I specialize in 20th century US military history, so while this period is close, it’s a little outside my sweet spot. The US military in the 1890s was no very different from the US army in the early 20th century though, or for any period pre-1940 except for periods of large conflicts that required an extra influx of men and material. Feel free to ask any questions and I will do my best to answer them!

A couple general answers as well as guesses.

To explain some of my logic, I'll start off with a few highlights of Richard White's discussion of the post-Reconstruction Army in The Republic For Which It Stands, the Oxford History of the US survey of that era.

"The postwar regular army of thirty thousand men managed to be both the American democracy’s least democratic place outside of a prison and a reflection of the nation’s hardening class divisions and growing inequality...African Americans, immigrants—mostly Germans and Irish—and the poor supplied most of the army’s manpower...Except for the ex-slaves, the army was a Northern and immigrant institution, in part because northern cities were where the army opened its recruiting offices. A person did not have to be desperate to enlist in the regular army, but it helped...Enlisted men’s pay might be a refuge in times of depression, but it fell well below civilian wages in good times.

(While it's not terribly applicable to this question, it's worth noting the officer corps experience was entirely different. Even a butterbar second lieutenant was in the top 10% of income earners thanks to deflation and despite glacial promotions lived luxuriously - importing frozen oysters via rail, for instance - in comparison to enlisted soldiers, who had terrible food and living quarters.)

But let's look a bit more closely at your great-grandpa. First, if he was indeed born in 1860 and did his first 5 year stint from 1882-1887, he wasn't 32 when he got in but 22. In that case, and knowing what we know about where the Army recruited, perhaps we should be looking further east for his accession and initial tour.

The Historical Register and Dictionary of the United States Army (Heitman, 1903) doesn't show a Fort Knight ever existing in this helpfully searchable database of military forts compiled here. I sniffed around to see if there was anything that could account for the letters you noted out West, and no dice. But there was a facility named after a Doctor Knight, Civil War surgeon Jonathan Knight, the massive and relatively modern 1000 bed Knight Hospital located in New Haven, Connecticut - the predecessor of Yale's current med school and hospital.

If that was the case, it would also explain a couple other things, like the handwriting that might say 'hosp. corp.' when he was up in far Northern California, the later transfer to San Diego (which always had a substantial naval hospital but also had a separate Army one - there were always concerns about Mexico), and why he may have remained a private: because he quite possibly was an orderly. This would also fit with the fact he was Scottish (and I take it Protestant), since that job would have been preferable to what most other enlisted men got to do at that point. The fact that his enlistments were also consecutive hints that he didn't run terribly afoul of officers along the way, along with being transferred by the Army several times (which they didn't really like to pay for if you were borderline.)

This would also clear up something else that nagged at the back of my mind as I read your question: precisely how did he afford to go West in the first place? Once the Gold Rush died down, direct transportation from the UK to San Francisco was far less common, expensive, and lengthy, meaning late 1870s Western European immigrants would generally sail to the East Coast and then take rail out West - which was not cheap either. (In fact, the rails provided subsidized transport for immigrants who bought railroad owned farm plots, with plentiful ads mentioning this in their native language in newspapers in their home countries.) Thus, if he was desperate enough to immigrate in his teens and enlist in the Army at 22, it's unlikely he'd have had enough saved to even make his way West on his own unless he'd somehow acquired some skills in the meantime. This would explain another reason for enlisting - to eventually get to the West Coast on the Army's dime - staying in until he got someplace he was happy to be (San Diego was a far cry from being near the Oregon border), along with getting out at the still relatively young age of 37 to proceed with his life and start a family in a region with a lot of potential.

Much of this is guesswork and I could very well be wrong on a lot of it, but I hope it gives you a basis for future research!

You can submitted the letter to the Smithsonian and have volunteers attempt to translate documents. Anyone can volunteer, we did it at work to get some “digital community service “. Was really awesome find.

Really awesome project they are doing, takes you back through time going through all the military type documents.

Here is the link. https://transcription.si.edu/browse

You can also google “ Smithsonian projects”, comes up number 1.

Could the Fort Knight be Fort Wright (Round Valley, Mendocino County)?

If so, then he mustered out of Fort Wright about about 17 years after the close of the Mendocino War, out of Fort Bidwell 19 years after the end of the Modoc War, and out of San Diego Barracks 17 year after the Calloway Affair of 1880. Those are the last major battles of the California Indian Wars near the respective posts.

It's not really clear what he was doing. It looks like he could have missed all the large California Indian Wars. There would have been a few skirmishes in the years between the end of the major battles and his discharge. For example, there was a conflict in 1887 in Round Valley when settlers invaded and commenced settlement on lands set aside as a reservation. But the feds were on the side of the Indian agent in that affair. It was the local governments that were working against the natives (and the Indian agent) in favor of the trespassing settlers. Following the creation of reservations, the role of the Army in Northern California was largely to keep the natives on the reservations and to protect them from conflicts instigated by the surrounding white settlers.

"Washington is a Long Way Off": The "Round Valley War" and the Limits of Federal Power on a California Indian Reservation Kevin Adams and Khal Schneider Pacific Historical Review Vol. 80, No. 4 (November 2011), pp. 557-596 (40 pages) University of California Press

Indian Wars Of The Northwest: A California Sketch Paperback – 1885 A. J. Bledsoe. Bacon and Co. San Francisco