I'm not sure if these are what you're looking for, since most of these are pretty innocent and people are suggesting pervy stuff, but you said it didn't have to be hentai --

Adarushan no Hanayome
Ojousama to Youkai Shitsuji
Nil no Koi☆Mahoujin
Chupa! (UEDA Rinko)
Full Moon Awakening
Living Game

Super Lovers maybe.
It's yaoi. Though the boy isn't that girly, the art and story are good.
Uke is 16 years old and seme 20 something if remember correctly.

In Marmalade Boy there's sth like that
also Kanojo wa Uso wo Aishisugiteru

朝露の少女。

“The longer we live, the more weight we carry in our hearts.”

koi kaze and Silver (FUJITA Kazuko)

older guy and younger girl-->
Dengeki Daisy ( boy 24 yo - gril 16 yo ) ( my fav )
Faster than a kiss ( 24 yo - 16 yo ) (my 2nd fav )
chocolate cosmos ( 25 yo - 16 yo )
love so love ( 25 yo- 16 yo )
ultimate venus ( 21 yo - 16 yo )
16 life
the king's pawnshop ( 23 yo - 16 yo )
ichiko jigan
hanayomesama wa 16 sai ( 21 yo -16 yo )
dramatic love album ( 23 yo-16 yo )
secret kiss
orange planet ( 24 -15 ) .(not sure )
paplillon ( 24yo -16yo )

Kore wa Koi no Hanashi is a good guardian x dependent story. The girl is in elementary in the beginning, but there isn't romance until a little later (within your specified age preferences).

Youko x Boku SS is also good (there's also an anime: Inu x Boku SS). High school girl, kinda fake loli. You specified you like H, and I know there's translated doujins of it out there, too.

Try Takane to Hana

Hyakujuu no Ou ni Tsugu!
girl 17 and old man around 30 😀

Mademoiselle Butterfly

Set before WW2 in Japan, Butterfly(her geisha name) met Chinatsu when she was 9 and he was like 20 something. She falls for him and gets together with him when she's 15. It time skips through the next 10 years (them getting married, her getting pregnant, him going off to war.) It ends when she's like 24.