I'm looking for a manga where the male lead is much older than the female lead.
It may either be any of this pairings:

1~ High School (Male) x Elementary School (Female)
2~ College Student (Male) x Middle School/High School (Female)
3~ Old Man (at least not exceeding 40's) x High School (Female)

*If none of the above fits your suggestions, it's okay as long as the male lead is older than the female lead.

I'm not picky on the genre. It may either be shojou, shounen, seinen or josei.
Anything that just crosses your mind would be a great help.

here are some of my favorites:
Ojikoi
Shuuden na Kankei
Hajimari no Niina
Keishichou Tokuhanka 007
Hirunaka no Ryuusei
Kiss
Don't Cry, Girl
Otona no Jikan

😁 😃

"The world is a dangerous place to live, not because of the people who are evil, but because of the people who don't do anything about it." - Albert Einstein

Next time use the manga tags ([m][/m]) around the series' names so we don't have to click on all your links... common courtesy.

Miriam
Miman Renai
Hana no Namae
Help!!
Jinbe

Love So Life
Shi ga Futari o Wakatsu Made
Meteor Methuselah
Hana to Akuma
Shiawase Kissa 3-choume
Kodomo no Jikan
Ikoku Meiro no Croisée
Kisu yori mo Hayaku
Lotte no Omocha!
Karei naru Shokutaku
Velvet Kiss
Ore ga Akuma de, Aitsu ga Yome de
Ichigo no Gakkou
Koi Kaze
Ressentiment
Rude 39

Another would be
Dengeki Daisy

check these links
http://www.mangaupdates.com/showtopic.php?tid=43464&page=1#post619102

http://www.mangaupdates.com/showtopic.php?tid=5702&hl=sensei%20x%20student

Hotaru no Hikari
Nocturne (Park Eun-Ah)

Antique Romance
Chocolat (kubonouchi eisaku)

Favorite Art:
Oboreru Knife
19 Days
Gangsta

I finally get what you said...thank you for the suggestion... >.<

"The world is a dangerous place to live, not because of the people who are evil, but because of the people who don't do anything about it." - Albert Einstein

100% Perfect Girl
Doesn't mention the age difference but I believe there is a couple of years between the leads.
One of my favourites but very dramatic.

Kinda surprised no one put Kore wa Koi no Hanashi - it's one of the better ones in my opinion.

Sugar Family -Step-siblings, sister complex, studentxteacher relations.
Ten no Ryuu Chi no Sakura -Fantasy, dragon clan, kidnapping...
Ikoku Meiro no Croisée...Cultural diversity...
Faster than a Kiss -Complete manga.. Teacher x Student.

White Dragon-Although it's from 1997 it's still nicely done both art & storyline. It's full of action & crime...
Meiji Hiiro Kitan -Mystery, some criminal element and merchant dealings.

Highly recommend...
Kore wa Koi no Hanashi -Author & young girl...
Taiyou no Ie -Childhood friends...

Newest manga for the ojisan-like age gaps is...
Kyou mo Uchi de Machi Awase -Child care & past neighbors.

I'm surprised I haven't seen Inu Neko Jump! listed yet. It's good.

Here's some other stuff that's pretty loliconish
Chitose Get You!!
Ro-kyu-bu!
NHK ni Youkoso!
Kodomo no Jikan
Nogizaka Haruka no Himitsu
These titles in the list all have anime, so you should probably try them too.

i'd like to suggest
hirunaka no ryuusei
dengeki daisy
takane to hana
love so life
p to jk
sensei kunshu
16 life - and probably some of aikawa saki's other mangas too... there are a lot

(If you're still looking for recs) Maybe try;
Mahou Tsukai no Yome
Gisèle Alain
Koi wa Amaagari no You ni
Cousin (more or less)

Seconding/thirding Kore wa Koi no Hanashi, Takane to Hana, Ressentiment, Meteor Methuselah, and Ikoku Meiro no Croisée.