Honestly, I think it depends on the relationship. Personally, I don't think I'd be particularly comfortable going above 8ish years, but like, love is love, right?
For some people, it's not an issue at all - despite the gap, they're at the same stage of their life, or want the same things, etc.
But, I think it's also an issue for some people that they don't necessarily realise. You can get pretty big gaps in maturity, particularly when you're both still young. When you've had a bit more life experience, I don't think the difference in maturity and the like tends to be as noticeable.
I think as long as both parties in the relationship are comfortable - then there's not a problem. But like, personally, I wouldn't be comfortable having a huge gap - what happens when you get older, and say, one of you is in their 80s and the other in their 60s? That potentially puts a large burden on the younger person to be a carer, or potentially, if you go by average life expectancy, means the younger person is potentially outliving their older partner by a fair stretch, which, y'know, can be really lonely and sad.
But yeah, in the end, I think the age gap thing is pretty arbitrary.