Well, I'm doing it anyway. (Watch out, Imma bout to go full homeschool on y'all)
The average lifespan of a rabbit in the wild is around 9 years. And the average lifespan of a human (in Monaco, because it makes the numbers easier) is around 89.32. So let's say 90 years.
At the beginning of the series, Good Ol' Master Smith said Heather was close to maturity. I think human wise would be 17-ish. Translating to 1.7 years old.
BUT, how does that compare to when a rabbit matures? Well, a normal rabbit is officially full grow around 12-18 months. So 2-2.5 years.
IT'S PERFECT. (lol, I'm looking these things up as I'm going along, so I'm surprised it worked out so well!)
So, Heather, at the beginning, is around 1.7 years old, close to being 2, which is full-grown.
That would explain why I felt they grew up really fast. Cause they did. Cause that's how bunnies work.
The only problem I can think of about this is that in book one I seem to recall there being a specific reference as to how many years have passed since Jupiter's betrayal, and I think it was 13 (correct me if I'm wrong here; I don't have the book to reference right now). Heather was alive back then, and she's not insanely old. I don't think the characters function fully like real-life rabbits (e.g. Heather and Pick don't have dozens of siblings), so I imagine their lifespan would be more human-ish, maybe a mix, since Jacks seems to have grown fast. It seems like something that Sam might leave vague in order to avoid discrepancies. Also it would be so sad if the characters really only lived a few years. That's just my opinion, though. (Homeschool right back at ya.)