Please explain to me: Why does he sound like a teenager?

I have been dating a 45-year old man with undiagnosed ADHD for a year. Both me and him are convinced that he has the condition. One of the many things that puzzles me about his behaviour is this: He mostly acts and speaks in a way that is relevant for his age - that is in a responsible, mature and experienced way. But suddenly he reacts, jokes and makes comments that is really embarrasing, like some teenager. To be frank, he sounds stupid and I really get turned of, I am in my forties and don't want to date a kid.

To give you some examples: He can comment on other womens breasts in a very immature, childish way. And he got very upset when his work colleague made a joke about him wearing reading glasses, which I am sure was just  a normal, friendly joke. He sometimes buys clothes that would fit an 18 year old better.

And then suddenly he sounds like the serious, experienced, mature man again.

I don't get this, and wonder if it is common among people with ADHD? Would be very interesting if anyone knows what I am talking about. 

(Sorry about my English, I live in Europe and speak another language!)