A yuri couple in yuri manga is, of course, no extraordinary.
We're given some different circumstances or problems of those yuri couple as they maintain their relationship together.
Some are sweet, some are in between (bittersweet).
Ruriiro no Yume [瑠璃色の夢] by Akiko Morishima in 2009 is an example of yuri manga which put in age-gap yuri couple/characters basically as their background information.
In this case, their methods to overcome the problems of the relationship unlike those yuri couple in the common yuri manga which heroines are in the same or not-too-far-different age.