If I were to hop into the TARDIS and travel back to ancient Egypt, would I find that the soldiers, military officers, scribes, and priests all somewhere between the ages of 13-19? I don't necessarily mean that ancient Egyptians were "teenagers" (esp. given the contextually relative nature of that term), since obviously they would be working, but in terms of their physical appearances? Or perhaps they would physically age quicker because of having worked outdoors?
Hi, I'll leave it for someone else to talk about demographics and maturity/behaviour in ancient Egypt specifically, but there may be some confusion about the meaning of the term "life expectancy", specifically "life expectancy at birth".
Here's a comment from /u/anthropology_nerd explaining life tables What was the ACTUAL average lifespan before modern medicine?,
... and here's a thread that links to a collection of posts discussing life expectancy in various cultures throughout history (not ancient Egypt, but you'll get the idea) How true is it that average human age was skewed by high infant mortality rates hundreds of years ago?