Actually, the one thing that Reginald Hargreeves had going for him was that he wasn't completely sexist. Given that all the children were born on the same day, there was no opportunity for seniority by age, only through the manner with witch Hargreeves acquired them from their biological parents.

Allison is Number Three.

But given that he doesn't say any of this, he'll never know. She also doesn't comment on Five's age. That's not her fun tidbit to share.

"Safety's never really been a thing before, so I don't know why it would start now." She's mostly sarcastic there. "But I'll figure it out."