Is it true that Imam Zayn al Abidin (as) got married when he was a teenager? If yes, was that the usual age of marriage for the other Imams as well?
The age of marriage according to Islamic teachings is as soon as the male and female need to be married. It is called now a days early marriage but it is in fact "the marriage on time".
The marriages of the Infallible Imams and their children were on time which is soon after puberty and was never delayed. In fact , it is Makrouh (disliked) to delay marriage.
Wassalam.