Mumsnet seems to take a very dim view of age gap relationships, far more so than I think the general population does. I've seen plenty of age gap relationships work just fine.
Every single time they're discussed here, though, someone wheels out the "rule" that you shouldn't date someone younger than half your age plus seven years.
People treat it like it's absolutely set in stone, and that breaking this "rule" is some sort of disgusting taboo. I've even seen someone argue it should be enshrined in law, with penalties for those who break it.
Personally, I've always thought it was complete nonsense. It was simply something in a French novel in the early 1900s. It doesn't really take anything important into account, and ignores the principle that consenting adults can do what they like.
Quite apart from anything else, why is it plus seven? Why not five, eight or ten? I'm sure it's only seven because that's a number that's had mystic significance since ancient times. Not a good basis for making laws!
Anyway:
YABU - yes, it's a rule and shouldn't be broken
YANBU - it's a big pile of nonsense and doesn't mean anything