My boyfriend and I have a 17 month age gap, and while it may not sound like much, a 19 year old and a 21 year old sounds a little questionable to some. Especially when the woman is older.
Everyone is pretty much in agreement that age gaps matter less the older you are, which I agree with. But I think it all comes down to maturity, really. Often I get the jokes about how my boyfriend is younger than I am, but they're only jokes. And, at the end of the day, it's only 17 months.
However, some age gaps can be more questionable. Especially those regarding older men favouring young women - and especially when considering the age of consent. In Hollywood, many younger women are seen with richer, older men. Don't get me wrong - some of these couples work out, but usually it's a different dynamic.
For example, Sarah Paulson and her partner Holland Taylor have a 32 year age gap - yet, they have been happily together for many years. And not many people have comments to say. However, Leonardo DiCaprio is always the one to pick on at award shows, with his infamous preference for women half his age.
So, I think it depends. As long as the relationship is ethically and morally good, there shouldn't really be a problem. Especially not in the eyes of anyone else.