1/2+-7 is a decent guideline for “most people won’t be creeped out by your relationship.”
I don’t think it should be law because love’s got a variety and “what’s criminal” and “what’s not tactful” aren’t the same.
That said, if I had an adult niece or nephew looking to date someone much older, I’d do my Uncle duty and do the obligatory caution thing.
But love makes people stupid and there’s usually no stopping it. So long as it doesn’t seem dangerous, I wouldn’t actively try to stop, although I might do a talk-thing with the elder in the relationship anyway.
As for me, I’m 45, haven’t bothered dating in forever. (stopped in my mid 20s after a few toxic relationships). I get offers (female and male alike) which is really flattering but I’d rather be friends. I don’t feel a need to cling or be clung to.
But if I wanted to, it wouldn’t be an issue. As far as age? If there’s a difference I’d take it into account just as I’d expect the other to do of me, no different than if I was an agnostic guy dating a Muslim or an American dating someone from India, or a guy with glasses dating someone with perfect vision. There’s always differences between people.
Young low level professors dating older undergrads isn’t uncommon but it’s at the edge of, “but whatever”. A 30 old’s range would be 22-46
Now if that young low level professor was going after 18 year old freshman, it’d look a lot different. Still, I can see what you’re saying. I don’t even necessarily agree with the guideline: it runs into flaws. But turning a common sentiment into a rule-of-thumb it’s close.
All this talk of ranges though seems creepy in general as it brings to mind grooming, which is a topic you rightly mention several times.