Interesting thread, it be great to repost what you felt were somewhat reasonable answers.
1) Literacy in the best case was actually much higher then any of us would believe. Pre-printing press (the most important invention in regards to literacy) Burma or Myanmar had literacy rates between 80-90% depending on the scholar you ask. This was because the buddist monks were extremely dominant, and took it on themeselves to educate every single child and teach every child how to read and write.
This is certainly the exception, but it is possible in a 'different' society. The relization needs to be that for such a high rate, there must be some body or bodies providing education.
This is not outside of the realm of possibilities in a fantasy setting where monks, wizards or clerics chose to play an active role in educating the young.
Age ratio's Children per Adults
My best answer is that it can very. But take a look at population pyramids. On wikipedia Afganistan is a classic example of a population distribution by age in a pre-modern society.
I can't find one that actually lists percents. or I'd tell you. basically though there are always about twice as many 0-4 year olds as there are 20-24, twice as many 5-9 as 25-29, excetera. Diviations would happen if there is a plague or similar catastroph or some major social change meaning people live longer. (Steadier food supply, more healing etc)
A couple would generally have 4 - 8 children, of these a few would likely die. (if they died early 0-5 - the most likely time for them to die- the couple would be more likely to have more kids)
It is not uncommon of for a man to have several wives all who die in childbirth. Having mutiple wives ( I am still talking mohogamy here) can push up that number, 5-12 would be a decent estimate.
If death of children/adults were less common (ie. lower infant mortality and other deaths due to healing magic) then the number of children would be lower, however the number of surviving children -- assuming that the fall in the death rate is not sudden, ie, suddenly there are more healers -- would remain about the same.
as there are no forms of social security, trust funds, savings plans, the only way Adults can garantee being look after as elders is if they have children willing to support them. Moreover children represent valued Labour in premodern times.