It's a lousy starting point. Depending on the age, a child will not be anywhere near as dextrous as an adult, never mind more so. All the ability scores of a child will be substantially lower than an adult's. Strength and Consitution will probably reach parity long before everything else.
-4 to physical scores, -2 to mental scores, Small size?
Originally Posted by Emperor Tippy
By level 20 though, you aren't capturing a wizard. A character lives to level 20 by being the most ruthless, lucky, capable, and paranoid bastard around. A wizard is throwing around a 30+ Int score and has, entirely in character, planned contingencies for his contingencies. He may well be running around with flat out total immunity to harm, he does not walk outside without an entire bevy of defensive magics around him and enough magic items to buy himself a nation.