The Mitzvah of Torah study:
Until what age:[1] There is no maximum age in which a father becomes exempt from teaching Torah to his son, and so long as a son has not studied the entire Torah and has not received the skills to do so on his own, the father remains obligated to teach him Torah irrelevant of the age of the son.
[1] Setimas Kol Haposkim who never mention a maximum age; See Admur Hilchos Talmud Torah 1:6 that one is to pay for his learning even after the son is married with children; Halacha 1:8 That there is a positive command to teach all the Talmidim, irrelevant of age.