Right around the age of about twelve to thirteen or thereabouts is the time in life when girls jump-start into puberty.
In a split-second flash and spurt of hormones, accelerated growth makes them tall and creates the spindliest legs around. Long, slender, very very skinny sticks upon which they walk and run, taking up about three-fourths the total length of their bodies. Or maybe even four-fifths so it seems. They can skip, jump and run very fast, that's for sure. Sometimes they skitter so fast in fact that the observer worries that these thinnest of spindles may bend and snap accidentally.
To imagine that in three or four or more years these bright little feminine spirits will have become blooming full-fledged sirens of the sea, making the naive and inexperienced boys of their age victims of random love and other chasing scenes. Survival of the fittest and the girls-become-women will win out, even if we men tend to fool ourselves into believing otherwise. Males meet their Anima, and females their Animus. Come together now and mix each other up.
As always, nothing less and nothing more. For the next adventure.