As others have mentioned, frankly this age rage, and age gap isn't that unusual and even just less than 100 years ago was totally socially acceptable. This was mostly due to, for a very long time in human history, life expectancy wasn't what it is today thanks to modern medicine. So marrying at an age like this was totally normal for the longest time.

However, if the ages were skewed any further, you'd be looking at possible pedo territory. Like she's 16, and he's 25+, though I wouldn't be surprised if this actually happened in history and they were perfectly happy with the arrangement. There's a whole grey area to this sort of thing, and then there's the obvious fucked up cases like you hear about in the middle east. Where a 45 year old man is marrying a 12 year old girl.

But in this scenario, I don't see anything fundamentally wrong with a 16 and 20 year old, because nobody would bat an eye with the same two people, but 20 and 24 years old instead. But this is strictly on a case by case basis, with the full consent and approval of their parents involved.