Is the prayer of one who prays behind an imam in his home valid, when he does not know if the imam was pre-pubescent or post-pubescent?

Scholars differ on the validity of a congregant following an Imam if the Imam is in the mosque and the congregant is at home, but able to follow him. As long as you doubt whether this act occurred before or after puberty, the default assumption is that you had not yet reached puberty, and that this act occurred before it. Therefore, you are absolved and nothing is required of you.