I always enjoy series where you get to watch the characters age, though they're rare. The funniest take on that subject is probably SEINEN 14 SAI. It's like 3 chapters in a 10 chapter book, with an extra (10.5) at the end. The first three chapters, with 10.5 tacked on at the end... if you enjoy comedy, do take a look.
I can think of 2 others: DOUSHITE NAMIDA GA DERUNOKANA but we don't get to see them when they're old, old men, just Junior and Senior High age to full grown and working adults (this author also has another series, but rather than aging forward, we instead see them younger through the series though flashbacks); and the other series is ....
Well, I don't remember the name of the series AT ALL, and I don't think I've seen enough manga by that author to be able to hunt it down. A male couple hooks up, split up because one of them got a girl pregnant, the couple hooks back up and raises a son. They own a coffee or pastry shop. Son's friend has a crush on the son. Original couple is VERY old, and the father becomes a grandfather... Surely someone will be able to supply the series name!
It would be nice to read more about this kind of love, the one that see both couple advance in age. Beautiful story and beautiful characters.