The human brain has actually not fully matured yet at age 19-20, so there is something to say for considering young adults a little more on the "kid" side. Of course this doesn't mean you can't act and be mature at that age. Obviously behavior differs per person, but the brain is still undergoing development no matter what, so you're not 100% mature and grown just yet.
Doesn't mean young adults cannot commit to a relationship with someone of their own age or someone considerably older, but it's more likely that it's not going to last forever. The younger party is still developing and undergoing changes, so chances are in a few years they will change their mind, want to try something else or are less compatible with the other, etc.
With a middle-aged couple this will no longer be relevant, obviously, but I think the age difference will start to matter more again once you start to shift towards old age. Age 30 and 45 doesn't not have huge difference mentally and physically, but 60 vs 75 does, or can have. Saw this with an uncle who had a wife who was 10+ older than him and who are now getting divorced. There was more at play there, but she did want to undertake less activities and just sit at home while he wanted to take up ballroom dancing again and was very competitive in that mindset and she struggled to keep up.
There's always the exception to the rule though. I'm not claiming every relationship with an age difference is doomed to failure, and I think one should be careful to spout there disapproval if a friend or family enters such a relationship. But there are more potential obstacles on the road. In the end it's up to the couple to decide whether they are willing to stick together and overcome those.
@ StillEden: maybe it's a confidence thing with your sister and her series of relationships with young guys? Could be that it helps her feel desirable, because if she can get someone at that age, she should be able to get anyone, right? I don't think it's really something you can easily discuss with her. Maybe if the current relationship breaks up again you could carefully bring up that you're concerned about her after all these brief hook-ups with guys half her age, but it's definitely tricky. If she feels you're being judgmental she will likely withdraw from you.