The recent dragon article (or was it Slayers Guides to Kobolds?) has them maturing in 2-3 years. Our campaign has already taken up 3 years of so of in-game time, so such a thing is not unplayable.
Is it me, or are kobolds the new smurfs? They aren't cute, they are vicious little xenophobes, the bastard children of a chihuahua, rat, and some sort of reptilian chicken.
That's rather arbitrary. The PC's are 19th level and have quite a bit of both arcane and divine magic at their disposal, and they are interested in keeping and attempting to raise the kobolds. Why shouldn't they be able to?
Give them no stats other than one hit point the first year. Give them half-stats for however long you want to say they're children. Races of the Dragon might have some insight on this, but I doubt it.