According to "reliance of the traveler" (the translation by Nuh Ha Mim Keller), to my knowledge the most important shafi'i manual of fiqh, prayer is fardh from the time a person enters puberty, as is fasting. Puberty is one of:
- the first wet dream
- the first menstrual bleeding
- or it is assumed to have begun by age 15 (by lunar calendar, i.e. the months of islam)
Thus the only way age matters here is that even if puberty doesn't set in by the age of 15, a person becomes obligated from then on. The determining factor is puberty.
You can find the rulings under the following headings in reliance of the traveler:
- obligation to pray: f1.1 (f1.2 relates the ages of 7 when a child should start being admonished to pray if it is able to eat, drink, and clean himself on his own, and the age of 10, when he should be beaten for neglecting it)
- obligation to fast: i1.3
- definition of puberty: k13.8
While this is from a shafi'i source, I have not seen any other opinions from any of the other sunni madhhabs, with the caveat that I do not know whether they also impose an upper bound on age, and if so, which one.