I've seen a few topics like this, but as of now I can't remember how to find those topics.
I just really want to read a yaoi where the seme is younger. and I'm talking about 5 to even 20 years younger. As long as the uke isn't really manly and big and so. I don't mind if the seme is a child or if he looks older for his age, as long as he is younger in age :> thank youuu

http://www.smackjeeves.com/comicprofile.php?id=135230
yaoi webcomic ._.

I'd recommend reading Naono Bohra's works, she often uses younger semes.

Some I like:
Ai no Kotoba wo Uragaeshi
Dare ga Omae wo Suki dato Itta
Boku no Shiru Anata no Hanashi
Gokujou no Koibito
Koi Shika Dekinaiyo
Second NAONO Bohra

Quote from DarkLemon

...but as of now I can't remember how to find those topics.

Here you go - Older Uke Younger Seme