on point: i grew up with a neighbor kid who was both small, and a late-bloomer in distance running. He was barely able to keep pace as Frosh in HS, but became top-20 in state XC by Jr year. So, in principle, it's not all over by 7th grade. Male physiology changes so much in the 5 years after puberty, it's hard to make strong predictions at 14. (Note: My state was a small one, so top20 doesn't mean THat much, and my buddy didn't end up running in college, even at his D3 school). BTW: smaller is generally better, Solinsky nothwithstanding.
more off point: I'd echo the bigpicture comments of others. But, i'd add the shared-enjoyment of a sport aspect. I was a lousy HS runner, but it was enough to give me an inside appreciation of the sport, and the historic resurgence of american running that's been occurring lately.
I barely had the talent to be the 5 runner on the HS XC team, as a senior. Mile PR 4:29, and i dove at the line. I did not have the talent to run in college, but I got a lot out of running even if i didn't win much outright (one track race, ever). I competed, worked on personal bests, learned a lot about my limits, and other people. And, the aerobic conditioning subserved other sports (Ultimate; figures i'd have talent at a non-scholarship sport) and activities (climbing, hiking). And now, like so many things i love in life (music, film, literature...) i'm a passionate fan of something i can't do clairvoyantly well.
I have a 10 year-old son. He's got lungs. I'm not so sure about the legs or the head, yet. Nothing in this world would give me greater pleasure that watching him win a race, or make the game-winning basket, or whatever. I'm in love with my son, and i'm a competitive guy, and i wanted to win when i was his age, and i'm not gonna be apologetic for wanting him to get that experience. But, realistically, the chances aren't that high, so i'll encourage him to try hard, and let him know how bad his old man sucked at a lot of things at his age, but that sometimes they were a lot of fun to do anyway. And in the Spring I'll get some tickets and ask if he wants to go down to Eugene and watch some really fast dudes race their asses off, and the crowd will go nuts as they come off Bowerman corner and he'll remember it, like it was yesterday, forever.