Rabbiofberlin, it is in the Gemora (Kiddushin, perek 4) — that any position of authority is included. A Rabbi is not that, unless he seats on a beis din. Then, I think, it depends on what is being paskened on.
Akuperma, it depends on the fellow’s intelligence and siyatta de’Shmaya. There are big Talmidey Chachomim, who had a late start.