25-30, if not somewhat older.
If He can say "...in whom is no guile," I suspect the object of the statement was old enough to have guile, and that having it would have been a problem.
They were old enough to carry out their mission of traveling two-by-two across the countryside and being taken seriously.
There is a distinct lack of references to their being young men to whom no one should listen. There is no reference to anyone being amazed that these boys were healing people.
Paul does not refer to being older than those others, just of having been selected late. And he was old enough to have been considerably educated and taken seriously by the elders of Israel.
Peter was old enough to be taken seriously by Cornelius, and to be accosted at the courtyard fire.
They were old enough to be taken seriously by those who came to arrest Jesus, and Judas was old enough to make a deal about 30 pieces of silver - and then hang himself.
John was old enough to be entrusted with Mary.
Matthew was a tax collector.
All these tend away from mid to late teens, and push the estimate more into real adulthood.