Generally speaking, if two people are just having a benefits situation or sex on occasion then I think so long as you're not breaking the law, age gaps are fine.

The water begins to get murky as you get into romance and long relationships because so many layers of power dynamics begin to be introduced.

Differentials in experiences and positions in life become more questionable as well.

Can an 18 year old really commit to a long term relationship with a 30 year old while not having the experience needed to know what they want? I don't know. Society will applaud that 18 year old for marrying another 18 year old while calling them too immature.

My point is age gaps are only complex when considering commitment or ability to commit. If it's just sex, it doesnt matter much.