Are you referring to Aisha from Into the Light Once More?
Technically, she's still the same age in her past life. Her past life she was a child though. So the age gap remains. It doesn't matter that she was Reborn. Her mentality is the only one that aged, it's still her own body.
However it's different when it comes to beings that are immortal or have longer lifespan, because they look forever young. If they are paired with a mortal being that aged differently (no matter the mental age), at some point the age difference in appearance would be reversed, as that partner grows older.
You don't want a 5 year old kid, that was in her 30s, to marry someone in his 30s, just because she's actually 30 in the inside, right?! That's why mental age should not be taken into account. Age gap, is still age gap
Another example of not taking mental age seriously & decide that age gap is okay because mental age being older:
A Guy in his 60s Isekaied to another world as a kid, have a crush with a girl that looks his age in appearance. Yet his mental age would be 60, that you could say that he's a pervert and a predator for liking a kid (￣∇￣") But appearance wise they are totally appropriate you know, they grew up together and go to school together.
... I've been reading comments with this thought everytime there's an update here. When clearly FL is older inside...oh well, I'm sure they don't have problem with that 'princess' who reborn as another white haired princess that is being partnered with an ancient being which literary centuries older than her ╮(￣▽￣)╭