He's still got a bit to pass my best of 19:03 (could never get under 19), but I didn't start running until I was in my late 30s

As for the original question, with good training and a focus on speed work (don't need a ton of mileage to get in a good 5K), I think it could happen in 6 months, but probably more like a year. That said, there are just some people that no matter how hard they work will never get under that threshold.