I have a lot of readers who are highschool - college age. In my comic, there are three main characters, two of whom are 20 and 17, and one of whom is this guy:
(he's actually much younger than he looks, but that's not obvious from the comic so far)
When I ran a "which character is your favourite" poll on Deviantart, the winner was DEAD EVEN between this guy and the 17-year old... which means that even among my younger readers, he's one of the most popular characters of the comic. It was really unexpected for me -- I love this guy but I didn't expect other people to care about him as much as I do!
It's worth mentioning that if your current comic is like..... idk, not to pick on BL, but if your current comic is BL and the main interest is young, attractive people discovering themselves, and then you switch to a comic with a protagonist who's 74, you are probably going to lose some of the subscribers who came for young, attractive characters they related to more.
What's important to you here, though? If it's getting and keeping subscribers, then yeah, don't do anything that might turn anyone off. But if it's telling the story you want to tell and connecting with people who needed to hear that story, then you might be surprised, like me, to find that there are a lot of readers who enjoy the same thing you do. In my opinion, I'd focus less on "HOW DO I TELL A STORY WITH OLD PEOPLE" and more on the character whose story you want to tell, and how to make their story compelling and tell it well!