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Morally, as long as the parties are over 18, whatever. I won't lie, I'd think it would be weird for some older person to be walking down the street hand in hand with someone who looked young enough to be their kid, but I'm not in the relationship, so it's not my business.

I think that with larger age gaps, it's not so much morally wrong, it's just that people tend to be in different places in their life. A 40 year old is going to want different things than a 20 year old. Even if the younger one thinks this is what they want, they'll look back later and regret wasting their youth when they turn 40 themselves and realize they're with a 60 year old and they're no longer a spring chicken. That's one reason why midlife crises happen. Unless someone is a gold digger and is just waiting for them to croak so they can get the inheritance.

History was different. Basically, you became an adult once you hit menarche or spermarche, and it was open field. Younger women were more desired because they were more fertile. It didn't matter if that's what she wanted. It was up to her parents, and older men were more well off due to more time to gather the wealth, so they were more desired there.

Things have changed now because women can choose their own partner, puberty hits earlier (and only religious nut jobs in cults will consider letting their 11 year old daughter marry anybody just because she got her period), women have jobs, and usually beyond cooking, keeping house and having babies, so she doesn't need a guy to take care of her. There is just generally more in common with those around your same age (usually about five years is the max), and those things can help build a relationship.

Sorry it took me a minute to reply; for some reason I didn't get a notification that you quoted me.
It sounds like you thought it was weird or whatever simply because it was different, and outside of your 'comfort zone', as it were.

Out of curiosity, what was it that made you uncomfortable? Was the discomfort caused by the actions of the younger or older person?
...which is why it's important to understand that argumentum ad populum does not mean that something is correct, just that it's a popular opinion.

For example, lots of people still think that vaccines cause autism, but they don't.
Discomfort =/= Immoral... though people often do conflate the two.

Again, it really all depends on where you live, since different things are more (or less) acceptable in different cultures. In some societies nobody would blink an eye at the thought of a girl being married to a guy 20+ years older than she is. In most western countries, it's considered "unusual", though I imagine that most people would be hard-pressed to provide a reason why they think that way beyond "because it is".