Been running for 6 years now. Still haven't run a sub-20 or even a sub-21 5k. And no, I'm not overweight. I'm 130 lbs.
I do run pretty low mileage by LRC standards though for two reasons. First, I'm a triathlete, so I can't run as much as someone who only runs. Not sure how much that holds back my running though. Second, I've struggled a lot with injuries whenever I try to build up my running. I do believe I could have improved my running quite a bit more if I hadn't had so many injury setbacks.
That said, after 6 years now, I think it's clear that there's a significant genetic difference between me and the 90% of respondents here who are saying that that they ran sub-20 5k in their first race as a teenager. I believe I have the potential to go sub-20 if I can ever train at the requisite level for that without getting injured, and I would die happy if I could run 18:45 someday (~1.5x the WR). But I clearly was not born with LRC-level genetic running talent. I.e. Unlike a lot of the posters here, there's no way I'll ever run sub-20 5k without doing a lot of training specifically for it.