Humans should be able to multiiclass, what's a commando, detective or assassin but a fighter/thief? However, when a human character is created, he must choose ot be MULT or DUAL classed. Half-elves live only a little longer so 'age' has no part in the discussion in terms of multiclassing.

Humans should get a 20% XP bonus, they have no special skills, but a drive for power/experience due to their short lives.