Most Helpful Guy
It would depend why. There are a ton of reasons why a guy may never have had kids. Some would be flags, some would not. Best bet would be to ask him about it, if it is a concern for you.
He might not have found the right woman, or at least a woman he wanted to have a child with.
He might have been focused on building a career first, hoping that he could find love later in life after he had put in the late nights that can kill a relationship.
He might have tried to have a relationship and build a career at the same time, only to have late nights and tight deadlines constantly get in the way. Maybe this is something that is no longer the case. Maybe it is something you'll have to watch for.
He might be unable to have children.
He might not want children.
He might have had a lot of confidence issues with the opposite sex, and found women his own age intimidating. You might be young enough that he felt safe to harmlessly flirt a bit, and then was pleasantly surprised when it actually worked.
He might have moved around a lot, making it hard to build something long term enough to result in kids.
He might have had parents that divorced, and then promised himself never to get married unless he was sure the girl was the one and only (this reason kind of pushed me and both of my brothers into being mid-30s with no kids and no wife, though we found what we were looking for in our 30s). And if he was only willing to have kids with someone he was married to, it's not surprising that it hasn't happened yet.
There could even be more outrageous ideas. Maybe he was struggling with his sexuality, before finally deciding he was straight after all. Maybe he has a very particular fetish, or is only attracted to people with a very specific set of criteria, that you happen to have.
Could be tons of reasons. As I say... if you are worried about it, ask him. He would be expecting the questions, I'm sure.