Pokemon Adventures is a non-canon spinoff that is loosely based on the games. Bianca has no official age.
It's the official-est age there is, and I think it's safe to assume it's the same. The BW characters at least certainly look older than 12.
The only game protagonist t be given a official age is Red (11), and the Gen 5 characters are explicitly stated to be older than past protagonists. Right now you're taking the word of what is basically official fanfiction.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod