That's because you're a sensible, mature and reflective person, Fink. When I was a student, one of my fellow students was in a relationship with one of her school teachers. We weren't that close, but from the few chats we had her home life had been awful. Her parents had divorced very messily and her relationship with her mother was toxic. This man took an interest in her and I suppose showed her what she interpreted as affection, so she moved out of the family home and in with him when she was 15 or 16. At the time we were students she was gradually realising what a huge mistake it had been and that he was abusive, and fortunately by the time we graduated she'd got away from him. I don't know how it all turned out in the end. I hope longer term she was able to find somebody non-abusive and settle down, or be happy on her own.
Men (it's mostly men, isn't it?) who do this seem to me to be looking for someone they can dominate. A woman of their own age will probably be too confident and assertive for them. They want somebody who will uncritically adore them and do what they're told. The immaturity of a teenager is to a man like this an attractive quality, not offputting as it would be to a sensible older person.