Not really much for under-aged characters. Neoteny (Run Faster, 121) is typically for changeling characters that look like their prepubescent metatype, and in part reflects this in a Physical Condition Monitor of 6 + (Body/2, round up). It's not a bad start to reflect a much younger character. Personally I ignore the +10% for custom armor, equipment, and lifestyle as IMO it makes no sense (someone on Dumpshock did some research and found modern kevlar vests for kids are no more expensive than for adults, for example). I would likely use the Physical Condition Monitor, but without any requirement of being a Changeling or extra karma.
Also, Attributes of 3 tend to reflect the human average (or racial minimum +2, typically, for other metatypes). Having a Body and Strength of 2 would be more typical for a young teen or preteen type of human character, which I have also seen in NPC stats of kids. Unless the kid is some kind of Doogie Howser or Sheldon Cooper, mental attributes would likely be more like 3's.
SR5's BRB, page 131, gives examples on skills and what rating is equal to what, relatively. Sort of. Going off of a better description from SR4a's BRB, page 119, and converting to SR5's skill range, a preteen likely would have no skill above a 3; maybe one at 4 for a skill of particular interest, and nothing higher unless the kid, again, is some kind of Doogie Howser, Sheldon Cooper, or has practiced incessantly for much of his/her short life (like an Olympic gymnast or figure skater). Considering the character's age, it'd also be very unlikely he/she would have any kind of vehicle handling skill at all (maybe Ground Craft 1 to represent occasional off roading on a motorcycle or ATV).