For elite talent-ID'ing purposes (and this is with the input of many international coaches), chosen at 5/6:
Physical:
Shorter than average, small body (not necessarily thin bones --> too prone to breakage).
Everything in correct proportions, although there's debate over this.
Wide shoulders, narrow hips (think Boginskaya).
Fast twitch - the jumps, sprints, reaction.
Looking at parents to get clues for what lies ahead, especially throughout puberty.
Character
Slightly twangy, annoying, flitty, fidgeting, bratty, cocky, showy - anything out of the ordinary.
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Me personally: I'll coach any kid, regardless of whether they're going to be the next Olympic champion or not. My only condition: that
they want to be there, and
they want to put the work in. I'll put the work in and they'll put the work in and we'll work together to get you to where you want to be.
I'd happily take a group of the "non talented" kids too - I'm living proof that there are kids who would never be accepted by talent ID (I would never have), however, if they work hard enough, they can still achieve. No one ever expected I'd make elite, that's why I love training so much and so hard: because I'm proving everyone wrong.
(And you're never too tall/old/heavy: it's all relatitive. I'm 20, 5"7, and I just got my giants

).