Depends on the game and how you're feeling.
Ranked? Sorry bud, pick someplace I'm not if you don't want me taking advantage of you being under leveled. (I might even suggest they duo jungle for a bit to catch up on experience faster).
Normals? I'll usually let them farm for a little bit to catch up so that all involved can have a more interesting match in 3 or 4 more minutes.
There are a surprising amount of idiots that will take the kindness of the second scenario, then start attacking me before they're caught up. I always take that as a signal "OK, you think you're good now"