My daughter is ~2 months shy of being 21. Her boyfriend is 46. For reference, I’m 45. She introduced him to my wife and me about half a year ago. Since then I’ve had to bond with him in order to maintain the great relationship I have with my daughter and also keep an eye out for her. But it’s tough, uncomfortable. It is what it is. As we say here “să fie ea sănătoasă” - as long as she’s healthy (health is the most important thing).
That sounds quite awkward and strange for you. I honestly can’t imagine how weird that must be. But it sounds like you’re handling it the best way possible by supporting your daughter, maintaining your relationship with her, and keeping an eye on the whole thing. She’s an adult who can make her own decisions (granted a very young adult), so from what I can tell you’re being a great dad!