Any age gap can be fine as long as both parties consent and neither are creepy people. (Consent includes being able to legally consent, i.e not a minor)

That said, anything more than 3 years already opens up a little bit of maturity gap, which increases with a growing age gap.

BUT - people with age gap of 0 years can still have massive maturity gaps, or gaps in interests. Having an age gap relationship may therefore be just as likely to open your relationship to those problems as having a 0 age gap relationship.