Can't really use anime as a benchmark for what age people hit puberty, and how they develop during that timeframe. (When people hit puberty, especially women, is largely dependent on their daily stress, among other factors.)

If Ascilia is ~10-12, then 1.x Minfilia would be ~15-17, which would make 2.x Minfilia ~20-22. She hit a massive growth spurt between 1.x and 2.0, while none of the other Scions aged from their 1.x appearance - because they know magic to halt their aging process (and apparently don't feel like sharing with Minfilia. Either that, or she's as magically inept as she is physically).