Remember, if you knew nothing else about it, wouldn't a 22-year-old man dating a 17-year-old girl make you worry why he's shooting for someone so young rather than a woman his own age? Just a little? The age gap is a more than believable source of conflict here. If she's asking herself if it's wrong for her to like a 22-year-old man (of course not), he must be asking himself the same question, and from his perspective the answer is not nearly as clear-cut.
Only we as the readers have the benefit of knowing he's good people and she's perfectly safe in his hands.