Continuity mistake: When Cogsworth gives the tour to Belle, he is walking a very far distance from Belle. Yet in a closeup, he is walking much closer to Belle.
Continuity mistake: Just before Gaston proposes to Belle, the band members at his wedding change between shots.
Continuity mistake: Early in the film, we see the enchanted rose, and then the camera zooms away from the Beast's room and the castle. We see that there are several aspects of the castle that change during the Beast's fight with Gaston. For instance, in this shot we see that there is no window and the room opens directly to the balcony. This changes later. Second, the space along the roof is not as wide as it later is, and definitely not wide enough to have a fight. Third, we see strange statues of creatures that later appear in the fight- but no long row of these creatures such as appears later.
Continuity mistake: In the stained glass introduction, the rose in the beggar woman's hand keeps changing. Sometimes she is clutching it at the top, sometimes at the bottom, sometimes pinching it with her fingers.
Continuity mistake: When we first meet the three blonde triplets, the one in the green dress has the Ariel-like bangs, but in the beginning of the Gaston song she has swapped hairstyles with her sister in the orange dress. When Gaston starts singing about eating eggs they switch back, but during the last shot of the song they're both sporting the red sister's hair.
Continuity mistake: When Mrs. Potts is singing during the song "Be Our Guest", the spot doesn't actually appear on her until she notices it.
Continuity mistake: Between shots of Gaston throwing the bag of gold on the table and the man taking a coin out, the bag opens by itself.
Continuity mistake: When the song Be Our Guest ends, in one shot Belle's right arm is on the armrest, but in the next she is suddenly clapping her hands.
Continuity mistake: When we first see Belle at the beginning of the movie, there's a bridge in front of her house, which she crosses over to get to town. But later, after Gaston plots with the insane asylum keeper, there's a wide shot of Belle's house with a horse and buggy waiting in front, and the bridge is gone.
Continuity mistake: When Gaston enters the West Wing with his bow, the painting on the right side of the door is different than it is at all other points in the movie.
Continuity mistake: When Mrs. Potts tells the Beast, "Straighten up. Try to act like a gentleman." the table she is on has nothing on it. Then when she says, "But don't frighten the poor dear.", two glasses and a batch of grapes has appeared.
Continuity mistake: In the scene where everybody's singing about Gaston, the legs of the chair he's sitting on change between shots.
Continuity mistake: When Maurice looks up at the castle after he has just arrived, a lion's head is carved over the front door. But when Gaston leads the villagers to the castle later in the film, the lion's head has been replaced by a green shield.
Continuity mistake: When Lumiere suggests to see "The gardens or the library" his flames are at his sides. Then when Belle replies "You have a library" his candles are at his sides.
Deliberate mistake: In the brawl, one of the villagers gets his teeth knocked out, but his teeth are intact again a few seconds later.
Continuity mistake: The cracks on the mirror seen while the Beast is taking a bath change between shots.
Continuity mistake: After Belle refuses to go to dinner with the beast, he bursts into the West Wing and we can see the hallway outside the doors. It's completely different than when Belle comes down it later.
Continuity mistake: When Belle sits down on a fountain to read the blue book she got from the librarian, some sheep pass by and one of them bites a piece of the page off. But in the next shot, when Belle sings "But she won't discover that it's him til chapter three.", the page is intact again.
Continuity mistake: After the wolves run away the Beast is shown standing in the snow before collapsing and there are five cuts on his arm. However, when Belle treats the wounds in the next scene, there are only three cuts on his arm.
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.