|You mean stuff like Merupuri?
- Rising Impact - not much in terms of romance until the ending
Spoiler (mouse over to view)
The characters in question get married, after a bit of timeskip.
- Dear Mine
- Please Save My Earth
- Soredemo Sekai wa Utsukushii
- Kumiko & Shingo - largly untranslated
- Otoyomegatari - while the characters don't fit your request 100%, it's a very good manga and definitelly worth a shot
- Bitou Lollipop - the age difference isn't that big