How long have you been with him? Have you seen any other red flags? If no, I’d pause before freaking out too badly. Maturity in 20’s happens at fits and starts. It could be he matured in the past 5 years. One note in his favor: he has an age appropriate partner now, and seems attentive. Was the age range appropriate for his previous relationships in the past few years?

I tend to take things head on, but would have no problem saying- hey, I saw some of your messages to young girls, and it totally creeps me out. What the hell was that about? When did you stop chasing teen beat? Did you not pick up the cues that it was probably a borderline problem when girls would respond “I’m 14”, like you should know better? And most importantly, is that something you’re still into, ‘cause that ship has sailed for me about 15 years ago, and I’m not gonna have angst about it.

If he displays self awareness and insight into how the situation- and he- has changed, I’d let it go, but have firmly conveyed that I had no tolerance for future stupidity, and it’s a one shot chance.