- Possible spoilers ahoy
- Abyss has no notion of time so use "real" Pandora time)
- All ages are Pandora Gakuen/Highschool ages because as Jun has stated, ages may be worked out differently in their universe.
Just think of it like this:
- 15 = physical age and/or mental age (how old a person looks or "is")
- 100 = how long they've or would've technically BEEN alive for in "real" time
For example Sharon looks 13/14 but is actually... *whack*, She has been alive for... (Okay, okay I'll stop)
There are many reasons, why some can look a certain age, but not actually be said age, most involving the Abyss in some form. Here are some we've actually seen:
- Entering the Abyss = Since the Abyss has no notion of time or doesn't anymore due to a certain someone's experiment (you'll never see this in the anime), exiting can bring one to a very different period of "real" time, eg. if I was enter the Abyss at say 1990, exiting it, I may end up in 2159, etc. This would mean that though I am actually **, I have chronologically been alive for 169 years.
- "Legal" contracting = a side effect of legal contracting for some such as Rufus, Break, Sharon, may stop their physical aging altogether but they still mentally grow older, and eventually will die. For example, though Break may look 24 due to his contract, he's actually 39.
- Double/Reverse/Quadruple contracts: Oz, Vincent,etc - Nobody knows wth these do; Possible reverse/inverse/non-existent aging, along with a host of other things.
- Being Chains = You're too badass for age
- Being the Intention = You're too badass for age,space,time and sanity
- Being Jack = You're too badass for everything except a certain woman...
I hope this rather comprehensive list answers your questions :)