Pop time is the time between once the ball hits the mitt to when it arrives at second base.
I have no idea what a good one would be for a 12/13 year old. Probably some where in the 2.6 to 3.0 range. I know that the top guys at 16/17/18 are some where around a 1.85 to 1.95.
catch to when the ball reaches second base. (pop of the catchers glove till the pop of the second basemans glove) For that age group I'd say anything under 3.5 is very good...Was at a camp last week where a college kid had a 1.8 pop time and every scout was drooling over it...Anything under 2.0 is considered MLB caliber..