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).
Become excellent friends with him. Push hard on the worst nostalgic millennial bullshit when you’re together, so she gets that younger generation cringe factor when she thinks about him. Seeing men as old dudes just like dad wrecks the lady boner quick, assuming that’s not her fetish.
Yes, the solutions to your parenting problems are emotional manipulation and treating your adult children like you have the right to determine their relationships.