Continuity mistake: There are 2 different designs for Gaston's chair that alternate all the time: one covered in dark fur with V-shaped antler armrests, and another with a lighter coloured fur with S-shaped antler armrests. Also, both antlers on each side of the backrest differ slightly.
Continuity mistake: When Maurice first enters the castle you can see that the carpet leading to the staircase is very long. When Belle enters the carpet is shorter.
Continuity mistake: After Lefou is soaked he disappears in the very wide angle of Gaston walking behind Belle. (00:07:33)
Continuity mistake: When Gaston says he'll make Belle his wife a woman with a grey colored hair suddenly appears out of nowhere in the close-up. (00:07:58)
Continuity mistake: When the three girls first see Gaston, the fountain is pouring loads of water into the bucket. A shot later the water has suddenly stopped and the girl in red has the bucket raised up to her waist.
Continuity mistake: When Gaston proposes to Belle he kicks off his boots, which disappear in all further shots.
Continuity mistake: During Be my guest song the menu suddenly disappears from Belle's hands between shots. (00:39:39)
Continuity mistake: When Mrs. Potts puts Chip into the cupboard, the handles change style and colour: from gold to black. Also, the carvings on the wood become blue painted for a brief shot.
Continuity mistake: Beast bursts into the West Wing, uses the magic mirror and puts it on the table. A shot later it has vanished. Note the shape of the rose between both shots and you'll notice that it's not an opposite angle. The mirror is definitely gone.
Continuity mistake: When Gaston stops Belle on her way home, the three blonde girls magically appear in the middle of the village sighing of love, then they disappear but reappear a shot later next to the last house in the village. (00:08:40)
Continuity mistake: When Belle finally walks home, the road is in the shadows. In the white angle the street is bright and sunny. (00:08:05)
Continuity mistake: When the three girls watch Gaston, the wall on their right has a chipping on the painting with visible bricks underneath. In the wide angle, the wall is fixed and the bricks are now visible in the second section of the wall.
Continuity mistake: Belle walks by the main street and the stalls, signs and number of people change between the close shots and the wide angle; check a blue pig-shaped sign called "Le jamon": a shot later it's round and brown colored.
Continuity mistake: During the Gaston song, when the three girls sing, the chippings on the walls behind and to the right change style between shots.
Continuity mistake: All along the asylum keeper scene, the amount of snow on the railing at Belle's home keeps increasing, decreasing and vanishing randomly.
Continuity mistake: After rescuing his father, Belle passes a cloth around his body and finally rests her hand on his father's chest. A shot later she is placing her arm on his back and the cloth is gone.
Continuity mistake: When the asylum keeper shows up, Belle's door has some black nails on the wood that disappear in the wide angle.
Continuity mistake: After Belle leaves the castle, rescues her father, and takes him home, snow suddenly appears on the stone steps to the house.
Continuity mistake: When Belle leaves the castle Beast leans on the window and howls. From a front angle he is nowhere to be seen.
Continuity mistake: After Beast lets Belle run away there's an aerial angle of her horseback riding and Beast howling with both gates open. In the close-up the right door is closed.
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.