Other mistake: There are two gears on the top of Belle's roof, but both of them are spinning counter-clockwise. (00:02:45)
Continuity mistake: In the scene where Maurice is sitting next to the fireplace, there is a picture with a silver frame on one of the walls. But after the Beast has burst into the room, there is a shot of the chair Maurice is in, and the picture is now in a gold frame.
Audio problem: When the horse comes galloping into the meadow, Belle cries "Phillipe" but her lips don't match what she's saying.
Continuity mistake: After Phillipe's reins get caught in the branches, the loop of the reins becomes longer between shots.
Continuity mistake: When the Beast bursts into the West Wing after Belle refuses to go to dinner with him, he grabs the magic mirror, sees Belle, and puts it back down again. Then another petal falls from the rose, but notice that the petals on the table are in a different position than they were in the previous shot.
Continuity mistake: In the tavern, which is the setting for the 'Gaston' song, right at the end of the bar there is a thick wooden beam attached to the bar and the ceiling. You can see this beam clearly when Le Fou sings "No-one's slick as Gaston...", and he prances around and then grabs the beam. However, right at the end of the Gaston song when they sing "So his marriage we soon will be celebrating...", the camera pans backwards out to the window, but the wooden beam has vanished. (00:26:00 - 00:29:45)
Continuity mistake: As Le Fou jumps on peoples heads during the Gaston song, there are three beermugs on the table. However, when he jumps on the table, and is then grabbed by the men, and is being swung back and forth by them, the beers have disappeared from the table. (00:26:20)
Continuity mistake: When Maurice comes to the fireplace the stairs are red, but when the Beast finds Maurice there and Maurice says, "I just needed a place to stay.", the stairs are now white.
Continuity mistake: When Gaston's mob comes and the dresser jumps on someone, she starts to jump down stomach first, but the next shot of her, she's feet first. (01:12:25)
Continuity mistake: Throughout the movie, the hour and minute hand that make Cogsworth's moustache keep changing sides, sometimes from one shot to the next, for example, when he makes Belle visit the castle.
Continuity mistake: When the Beast brings Belle into the library, there is a huge globe on the ground, but in the next shot, as Belle passes over it, the globe is much smaller and is now on the table.
Continuity mistake: In an overhead shot as the villagers enter the castle, there is a blond man in a brown coat carrying a torch. A second later, as the brawl begins, this man's torch has been replaced by an axe.
Continuity mistake: When Belle enters the bookshop, she pushes the lower part of the door open. When she leaves it is closed and she opens it again. (00:04:25)
Continuity mistake: In the very short scene where the Beast throws Maurice into the machine and it carries him away, the front entrance of the castle is utterly different from normal. For instance, normally there are no stairs leading up to the castle, the statues on either sides of the doors are different, the walls next to the entrance are shaped differently, and so on.
Continuity mistake: In the scene where Lumiere and Cogsworth are persuading Belle not to go to the West Wing, the width of the red carpet on the stairs changes, it becomes much wider between shots.
Continuity mistake: When the Beast brings Belle into the library the books on the table change between shots.
Continuity mistake: When Belle returns the book to the librarian, he puts it on a shelf, and it is leaning against other books. A little later, it straightens itself without the man touching it.
Continuity mistake: When Belle goes to the West Wing, from an inside shot, after she opens the door, there is a handle on both of the doors. It then cuts to a different angle and pans backwards, and now there is only a handle on the left door. (00:42:55)
Continuity mistake: There is a scene with Belle and Beast dining. Beast wolfs down his meal out of the dish and his face is covered with food. Belle looks shocked, so Beast then tries to use a spoon and he is completely clean. Belle smiles sympathetically and Beast is covered with food in the next shot. When they are both eating directly from the bowl, Beast is clean again. It is the same meal. The food is missing from his face, although not an uncommon thing for animators (especially of kids films) to do on purpose, Disney probably wouldn't consider it a mistake, but rather, a convention, to make the characters always look their best. (00:51:30)
Continuity mistake: When Belle arrives at the Beast's castle, her cape hood is down, hanging off of her head. In the very next closeup it's up on her head. (00:20:40)
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.