I don't know why either. I like most of the younger men who come to my room. As long as they tell me they're 18 or over, we're on. I do get a lot of them who ask if I mind that they're younger, so maybe your experience isn't isolated. Maybe a lot of younger guys feel that way. I've had some younger guys who behave as RoseCavilla stated, and many who don't.
One thing a lot of younger men do that older men don't is ask for mommy/son roleplay. I've gotten better at simply saying, "Sorry Sweetie. That's against Streamate's rules. I can do stepmom, but nothing incestual." Some leave, some get a private, and some just get silent. Maybe that's why some women start to send a weird vibe.