I'm more inclined to go with an "18-19" year old dominant girlfriend with a 20-30 year old submissive guy. Like, it started off as him not being submissive, but sort of regular, and he loved the fact this young up an coming 20 yo, dote on him. But then, she used that to gain his trust and turn him more submissive while she started to dominate him in every aspects of his life... and because he had troubles finding a girlfriend, the fact that this one started out being highly affection to him, that he ended up conceding to her every demand...
Or something along those lines... someone forever changed, the whole switcheroo... she posed as one thing only to slowly change him to what she wanted, without him even knowing it. In the end, there was no turning back for him *grins*