Great that nothing happens with Nina but considering he has like a 10 year age gap with the head maid it's still weird...
I'm pretty sure they mentioned all those maids were teens. (If I'm wrong, I'll be happy) And no, the whole "but the other world thinks it's ok" makes it ok cause that's just the author talking. They could've made the age of adulthood any age they wanted. They chose teen cause that's what they wanted. It's a fantasy world for Pete's sake! The author makes the rules.
Ch.11 page 8. It says that she looks 20. There's no problem.
All the OTHER girls are teenagers.
This is not a harem. A guy gets reincarnated into another world with dimensional space magic, including teleportation magic. After meeting a nice family in a rural village he decides to live in the village while working in the capital. There is romantic tension between him and his head maid. (She manages his other house in the capital.)
Also, don't listen to SweetBrownBabyDoll. The MC clearly doesn't have a thing for little girls. He's essentially treating her like a little sister.
And again, romantic tension with Head maid.