I've been wondering if there is any shoujo/josei/seinen that fits this criteria.The girl should be in middle school and the boy in high school (or more but I don't want lolicon).The ideal would be the girl in her last year of middle school and the boy in the last year of high school.What I'm looking for is a "mature" romance.I don't mean it in a sexual way (though I don't mind that at all) but in the way the characters are portrayed which is why a seinen would be good as well. 4i2g5s

I'm not sure, but Kore wa Koi no Hanashi and Shi ga Futari o Wakatsu Made (Seinen) have so far been well-handeling the topic of the age gap between the characters, even if the latter one isn't tagged as romance and the male counterparts are already adults.

"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

Shugo Chara! (not mature)
Youko x Boku SS (He's a bit older)
Hana to Akuma (This is a definite read!)
Mei-Chan no Shitsuji
Antique Romance ( Mature, but not smutty)
Immortal Rain (he's 624, but died when he was 24 thus a tad bit older than you are looking for but she's 14 at the beginning (Innocent as hell so not as creepy as it sounds))
Miriam
Mademoiselle Butterfly
Yume Miru Taiyou

As they played the game...I waited for someone to call "onigiri." But no one called. I was very little then. I had almost forgotten about that. [...] But they knew...there would never be room for an onigiri...in a fruits basket.
-Tohru Honda, Fruits Basket

Thank you for you recommendations.

I only read Antique Romance since it was the only one that fitted my criterias and it was exactly what I was looking for.If you have others similar to this one I will gladly read them.

Actually, Mademoiselle Butterfly is a lot darker than it looks. It's quite moving!
Personally, I sobbed buckets.

As they played the game...I waited for someone to call "onigiri." But no one called. I was very little then. I had almost forgotten about that. [...] But they knew...there would never be room for an onigiri...in a fruits basket.
-Tohru Honda, Fruits Basket

I second Mademoiselle Butterfly. It's very heartwarming. 😃

"'The lives I've taken will be paid for with my own life.' That's one way to look at it...but it doesn't mean they come back once you die. Lives can also be paid for by the sword; by redeeming yet other lives. That is why 'Hitokiri Battousai' is still alive in the Meiji Era."

don't click this link...