I think she fell in love with him while he was still 9. The Christmas special was very telling, especially the way Rose was after the regeneration, I've always thought her shock/disbelief and upset at his transformation was fuelled by subconscious romantic feelings for him...
However, I don't think The Doctor had any kind of feeling beyond friends until 10. I think the regeneration definitely changed their dynamic and come series 2, there were definitely vibes of something more than a friendship between them.