If it's going to be the same group of characters, it'll probably be better as a single series. Maybe you can organize it so that the time progresses naturally?
You could also just have the gimmick that these are characters who have a friendship that lasts decades, and you just do slice of life stories bookended by their elder selves reminiscing, or something similar. If the character designs are consistent enough, the audience should be able to follow it with just a bit of narration here and there.