When I was in the military I saw lots of big guys (200lbs plus) with a sub 23-22 minute 5k time. There were even big guys with a sub 42-43 10k. We were just forced to run regularly.
As for age, maybe it makes a difference when you're pushing 60, but other than that psssht.
I know too many personal examples to believe age is a restriction, especially with all the training and nutrition info we've got available to us these days. A muay thai instructor I know is old school, pushing 50, eats normal food, takes no supps and can easily outpace, dismantle and rip apart younger, fitter, buff looking students. Now he has mentioned he feels the effects of his age at times, but his training's kept him at an above average level. Maybe he's a little slower than a younger version of himself, but that's it.
An ultramarathon runner I knew of from my military days was 40s, and had a 36 or 37min 10k time. That's just a couple examples. I've personally seen tons more. I'm not really into distance work anymore myself though.
I think its all training and mind.