I don't find it hard to believe at all. Here's an incredibly unscientific way of looking at it:

I'm 27. Among me and a hastily-assembled list of 14 of my male friends, 7 of us definitely have at least one sports betting account, 4 definitely don't, and I'm not sure about the other 4. I'd bet (heh) at least one of them has an account.

It might be more informative to see how many men actually use their sports betting accounts. Technically I have an account, but I haven't used it in over 2 years. Won a bet that the Heat would beat the Celtics in the conference finals, realized I was now net-positive by several hundred dollars, cashed out, and uninstalled the app. Never looked back.