I have some rules. Not to try to change the laws or brutally judge others, but they're at least strong guidelines I live by. I'm not yet middle aged, but getting there.
Late teenagers should be left to their own ages, within individual needs and good reason. Even if legal, a 16 year old should be with other teenagers, and its not a great sign if they're even with 20-22 year old. However, a 19 year old may date somewhere up into the 20s, but the higher it goes, the exponentially more risky such a relationship seems. I definitely do not think its a great idea when I see public figures with 18-20 for the younger, and with ~35/30s for the older (see: Dustin Lance Black), but that gap is the very very biggest for my tolerance, and I hope the younger is relatively mature about it, and the older is exhibiting a ton of responsibility aside from this detail existing. You should not be with a teenager as a clearly middle aged adult - period. Go be whatever else, like a platonic mentor, friend, or sure, later on, explore each other sexually. But a partner? Kind of ridiculous.
Early 20somethings are still mentally developing, and arguably in many other ways. They're not kids, but sometimes its going to feel that way. Even if a 20 year old is totally independent, successful, and graduated or already on career path, I still think they should be with someone not middle-aged, and ideally only with 20somethings. Of course there's surprising exceptions, but I don't think this is the time for someone still quite immature to be with anyone who could be their dad. However, going through the younger 20s, I think things start to really open up, and by 25, I think there's a lot more freedom that should be afforded and respected. If a 25 year old wants to be with someone middle aged, I wouldn't advise it, but I now have a lot more confidence they know what they want and what their relationships are. No guarantees, but it seems much more likely. So go date that 35, or 45, or if you insist, 55. But I also think they're too young to chain themselves to a retiree or whatever. At the very least, go date working age adults, and ideally, still stick to younger adults.
By 30 I stop giving a shit and if they want to be with middle agers, fuck it go for it, and if they want to be with seniors, then its their lives but I don't get it.
In my 20s, I developed my upper ceiling of interest from 30s to 40s, not 50s+. Now in my 30s, my lowest is early 20s (and I even doubt that now) with my ceiling being 40s and very unlikely 50s. By 40 I'll probably be completely disinterested in early 20s, and by 50 I may be completely disinterested in late 20s. When people get to be over a generation older/younger, I don't see much of the point besides MAYBE very casual encounters, and even then, I'd feel gross about being any part of a not-totally-developed adult's life (aka under 25).