Both of my DDs are very athletic, but I have only really thought of the younger one as an athlete. I think that's because of her attitude - she strives to improve, she likes to compete and she works really hard. My older DD is the opposite - she has a fair amount of gymnastics skill (they want to train her for the team), but she doesn't have the spirit. In the end she decided to stick with ballet - it suits her personality better, and she's really good at it.
Both have had a high degree of control over their bodies, from the time they were babies. Both have very well defined muscles - my younger DD has 6-pack abs and she's only 7! Younger DD has incredible self-preservation instincts e.g. when she was learning to walk, she'd fall backwards from time to time, but never hit her head - she fall onto her bottom, momentum would send her onto her back, but then she'd avoid having her head hit the ground by doing an awesome ab crunch as she was falling! Another thing she did that was amazing: she was at the top of a tall playground slide when she was about 18 months old (I let use it because she was so capable), but this day she was playing the fool, kicking one leg out to the side while sitting at the top. I spoke to her about it, and was just approaching the slide to make sure she was listening, when one kick overbalanced her and she flipped over the side. Almost any other kid would have fallen straight down to the ground, and that's where I was looking for her as I ran to the slide - but I actually found her at the TOP of the slide - the little monkey had shot out her hands and grabbed the edge as she fell, and instead of dropping to the ground, she pulled her body back up from a hanging position! The day I saw that I knew she was truly athletic and was going to do great things in some sport. I also never worried about her again on that slide, and she's been using it without incident ever since. This DD is shorter than all of her classmates at school, but wins every race on sports day.