Well, I know of a couple even just here on Tapas:
Office workers in their 30s are pretty common in BL in general. One particularly good one (that even has a live action drama adaptation on Crunchyroll) is:
-Cherry Magic! 30 Years of Virginity Makes You a Wizard??
A couple comics I know feature protagonists in their 40s are:
-40 x 40 Chikuwa Kaigi
-It Can't Be Mutual Love!!
One that I can think of that has a protagonist who I'm pretty sure is in his 50s is:
-Suki ni Nacchau Yo
That's all the specific examples I can think of off the top of my head, but I read a lot of single-volume comics and then immediately forget what they were called lol.
Stepping outside of BL comics specifically, I can also think of several movies from around the world that feature older gay couples. More than just The Birdcage.
-Four More Years
-An Almost Ordinary Summer
-Love is Strange
-Ideal Home
-Uncle Frank
-Twilight's Kiss
(I remember more movie titles because I track the movies I watch more carefully than the comics I read lol)
It's not actually an uncommon theme! I tend to seek out stuff about older couples, so there's definitely an audience for it if that's what you want to write.