This is a harder answer I think than it looks. Legally I agree that a 21 year old can sex up all the 50 year olds they want and vice versa. I also agree that there is nothing *inherent* in 21 year oldness or 50 year oldness that makes relations between the two immoral. All that being said... in *practice* a 40 or 50 year old person has gone through stuff in life that a 21 year old just plain has not. I don't mean just experiences in general - under that I'd have to think that, like, 35 year olds shouldn't date 40 year olds or whatever, and I don't think that's true at all - but in our culture 21 year old kids (and sorry, but to me most of them are kids, as I was when I was that age) simply have not gone out and experienced what it's like to have, for the lack of a better all-encompassing term, life happen to them. There are, I feel, things that you have to experience - understanding what it's like to need a job to pay your bills and not have one, understanding what paying bills means and the real consequences of not doing so, getting (and maybe even losing) that first job, falling in love with someone and being rejected... if it all adds up to anything, it's... just generally realizing how small a part of the world you are. Okay, I guess there was a term for what I was looking for after all, and that term is "humility". Sure, some people never learn this even in their 50s, and some 21 year olds have faced all of this for, like, 5 years by the time they're old enough to drink. But there is, I feel, a very specific barrier that I think you have to cross over in order to be able to relate that happens a little bit after that age for most people, and if you're not relating with the person you're sexing up or whatever, what is it that you're even doing?
The other issue I see is that stuff like this rarely happens in a vacuum. 40 and 50 year olds, just by dint of the fact that they've been around for longer, often find themselves in positions of power above 21 year olds. While, again, it's not necessarily a *feature* of 21ness or 50ness that makes those power dynamics important to acknowledge, it is *absolutely* immoral, I feel, to disrupt that dynamic by sleeping with a student or new employee who you are the boss of. There's also at some point some level of unequal footing that's going to happen between a lot of people who are older and a lot of people who are younger in terms of relationships because the former have been in more of them but that's hardly a universal thing - you can be inexperienced at 50 too, and just because you're experienced doesn't mean you have good experiences - and I think that's for individuals to figure out. And finally, I *think*, younger people tend to be more malleable of thought than older people, but as a middle aged person who at least tries to not be set in his ways, the fact that a lot of people my age are that way tends to make me, anyway, more attracted to (slightly) younger people, not less so.
I don't think there's a "half your age plus 7" rule that makes sense, honestly. There's just a "am I mature enough for this person, and is this person mature enough for me" question, I think. All the stuff I talked about above are trends, not hard and fast rules.