When the oldest children born to superheroes are about twelve, it therefore follows that even if the superheroes refrained in the first years, when they did not know how it would affect their children, it's about twelve years plus a little wiggle room since the heroes started to appear.
Therefore, the origin story of a superheroine, who was not in the first batch, is about twelve years before that of another. Putting her at most in her twenties when another superheroine has her origin story. I can still see her talking with the second heroine because of a certain commonality, but she's going to be a different sort of mentor.
Hmmm. Maybe another character in the first story can be the older woman mentor. A heroine of over a hundred could be easily produced by having her gain powers at ninety.