So long as both parties involved are [or are above] the legal age of consent, then anywhere between 5 and 10 years difference is fine.
[Maybe even a 15 year gap.] But in stories, if I was personally writing one with a an age gap… I’d do an age gap of 10 years maximum, but that’s just me.
I remember when my cousin [female] was 15 and she dated a 42 year old guy, I was absolutely disgusted! That was so wrong and I tried to tell her that he was basically a paedophile… but she wouldn’t listen, she was in her rebellious stage at that point… and anyone who didn’t agree with her was against her, or so she claimed.
Even though age of consent is lower in England, than in the USA [it’s 16, here.] she was only 15… so not only were they breaking the law, but that made him a paedo! It’s the facts.