How old is too old to date my Stepdaughter?

I'm trying to figure out the best way to explain this. My 16 year old stepdaughter (she'll be 17 at the end of Sept, and a Junior in HS this year) started working her first summer job this year and had met quite a few new people. So older and a lot about the same age. She had gone out with a group of her coworkers to movies and things and it was never a big deal (she's quite responsible, good grades, good kid, never had any problems with her at all). Her Father and Step Mother are a little too strict and my Wife isn't strict enough (IMO) due to an overly strict upbringing herself. Anyway two weeks ago she starts talking about some boy she works with and it becomes a thing were they're going to go to the movies (which wife said was OK) then my SD wanted him to come over and hang out one day (wife was home and I was at work) and it wasn't a big deal. Turns out that this guy is 19. When questioned about it my SD was not very forth coming with answers to questions I had ( either she didn't know or would mumble a bit). So my wife gets a call from her ex telling us this guy is actually 20 and after meeting with him says it's OK to go out with him but she can't be alone with him. I'm not OK with this at all and told both of them this and they kind of down played it. So I dig a little and come up with this guys birthday as August 13 and he's going into his Junior year of College. Can't find a year for his birth and Step daughter doesn't know. So he's either 19 going on 20 (which is pushing it) or 20 going on 21 (which is way too old to be seeing a 16 year old girl. Anyone have any advice? I just don;t see why a guy/kid that is that old would want to date a 16 year old girl. While the only couple of reasons I can see her having is either guys her age are to immature or the fact that she can brag about her College aged boyfriend to other girls when she gets back to school. All in all I hate that my opinion does';t matter to anyone else in this matter. Sorry for my venting

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