I once went to a pidyon haben for an adult male – he made it for himself. He became dati as an adult, and found out he required a pidyon haben but had never had one. I will always remember this one!
When we did my son’s Pidyon HaBen, there was a Russian person who had a safek as to whether or not he needed a Pidyon. He asked us if he could do so at our son’s (since we had the Kohen there already). We said yes (with the Kohen’s permission, of course) and he did his own Pidyon (without a bracha) right after our son’s.
The Wolf