I was 6 foot @ 250+ lbs at 17. I use the word "kid," informally to identify him over the rentacop. I even stated he was a 6 foot tall football player, so he could have taken the guy down. There is no doubt about it.
at 17 I was 6'2" about 280 LBS... I could have easily taken an older person down, him being an athlete gave him an advantage over another average teenager.. but that's neither here nor there. The article Kelsey put up states that Zimmerman shot upwards to Martin while he was laying on the ground, this puts a whole new twist on the situation.