I was reading that the average life expectancy in early modern agrarian countries in Eurasia was around 25-30

I'd like to make a general comment regarding this. Often when people talk about 'life expectancy' they are talking about 'life expectancy at birth'. That last part is important. Perhaps counter-intuitively, as you grow older your total life-expectancy goes up.

The average life-expectancy at birth includes all causes of death at all ages. Dying from old age (whatever 'old age' is), dying from the plague after your prime, dying a violent deathin your prime, dying from a childhood disease, even dying shortly after birth.

This means that the average includes babies dying in their first year. Now if someone is born and they manage to make it to their first birthday, they haven't died in their first year (obviously). This means their odds of getting past the average life expectancy at birth has gone up. Their life expectancy has increased!

This principle applies throughout someone's life. Surviving another year means you didn't die whereas others included in the average did. Make to age 10? Congratulations, you beat the childhood diseases- your life expectancy now has increased a lot! The longer you survive, the longer you will continue to survive, statistically.

For most of history, childhood mortality was very high. This lowers the average life expectancy at birth by a lot. What this means is that people making it to child-bearing age (say 15) - thus who didn't die in childhood - have a higher life expectancy. People making it to that age had an average life expectancy of 50 to 70 (this is a general statement - it varied in time and place and was affected by things such as social class and major epidemics).

Sorry for not commenting on your question about orphans. Unfortunately I know nothing about that. I did want to make this remark regarding life expectancy though.