Mauril |
Fly is already a class skill for all sorcerers (including those with celestial bloodlines), so they get the +3 bonus as soon as they put a rank in the skill. They cannot put a rank into the skill until they have a reliable means of flight, which is specified as a spell or other special ability. So, since Fly is a 3rd level spell, assuming your sorcerer takes it, you can start investing in the Fly skill at level 6.
They also gain the bonus for the Good maneuverability at level 9.