If the functions and rituals are conducted at home if the priest who conducts the rituals and functions happens to be younger than the kartA, can the kartA prostrate before the priest while offering sambhavana?

KartA can certainly prostrate the priest even when the priest is younger than him while offering sambhavana.
Srivaishnava-s can certainly prostrate before another Srivaishnava who is younger. But abhivadanam is not performed before a younger Srivaishnava. And he will also not do the pratyabhivadanam. Pratyabhivadanam means blessing the other person. But one is justified in prostrating a younger Srivaishnava.