I’ve actually seen a couple of people I know have a youthfulness and zest for life brought back to them from having kids at a later age. Of course, they were both in great shape, and the wife was younger, but it does happen. Unfortunately this is rare now, because most people in general, no matter the age, are lazy unhealthy fat fucks who don’t take care of themselves. I say as long as you can chase your kid around and do physical things with them until they’re 14 or so, you should be good. That means that if a man stays active, healthy, and fit he should be fine until 45-50 IMHO.