Other mistake: There are two gears on the top of Belle's roof, but both of them are spinning counter-clockwise. (00:02:45)
Other mistake: In the dance scene, when Mrs. Potts says "true as it can be", in one frame her eyes turn black and then turn blue again. (01:00:25)
Other mistake: At no point in this movie is the background of a room ever reflected in a mirror - even if the characters themselves are reflected in it.
Other mistake: After Lefou is stabbed in the butt by Cogsworth, there is no hole in his pants. This is impossible, since the scissors did pierce him.
Other mistake: When Maurice is gathering all his belongings so that he can go out to look for Belle, the lantern is not reflected in the mirror.
Answer: In the prologue, it states that the Beast must find true love by his 21st birthday, and that he had been cursed for ten years, so since his relationship with Belle takes place as the deadline approaches, he must have been 10 when he was cursed.