There is a comic that came out a couple of years ago with Minsc, in it I think it was that he had been turned to stone and was used as a statue. When released from his petrified state he and his trusted companion Boo continued to fight evil.
Not sure at what date in the world this occurred, but it could explain why he isn't an old man.
Found this " Sometime in the 1480s DR, the Beloved Ranger was accidentally struck by a wild magic surge caused by the wild mage Delina, as a result of an attempt to cast a spell at an attacking gargoyle. The surge dispelled the petrification effect, returning Minsc and Boo to life. "
