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.
Continuity mistake: After Beast tells Cogsworth that he lets go Belle because he loves her, Cogsworth tells everyone, Chip standing in front of him. A shot later Chip is by his right side.
Continuity mistake: Beast transforms into human and says "Belle, it's me." As of then his hairstyle changes in every shot: First it's long and falling behind his shoulder. Then it partially covers his right chest, and when Belle caresses his face his hair is tucked inside his shirt.
Continuity mistake: After Beast becomes human he flies, then falls and once on the floor check the right side of his leg touching the stone floor. When the angle changes his shroud is now covering the floor.
Continuity mistake: When Beast is about to die, Belle's left arm is on his chest. Beast raises his right hand and then Belle's arm is gone.
Continuity mistake: When Lumiere becomes human the marble columns and golden ribbons by the wooden lattice disappear.
Continuity mistake: When Lumiere becomes human the roses behind swap from light pink coloured to dark pink and red.
Continuity mistake: When Beast finds Belle in the West Wing room he shouts and she backs up to a nearly destroyed wardrobe, but in the next shot the style of the wardrobe behind her has changed and it's in perfect condition.
Continuity mistake: When Belle opens the door to the kitchen there is no shelf on the wall beside the door, but in following shots one has suddenly appeared with items on it and more things hanging underneath, which changes when Cogsworth lands in the bowl.
Continuity mistake: After leaving the book store, in an overhead shot Belle passes a shop just as a woman is pouring water towards the downspout, and in the next shot the shop suddenly has a large hanging pipe sign and a table under the window, the green shutters have vanished on the right window, the wood trim has changed, etc.
Continuity mistake: In the overhead wideshot right before Maurice walks in there's a green mat by the doorway, and nothing hanging over the large window, but when Maurice walks into the tavern there's a blunderbuss hanging over the large window, and the mat has vanished.
Continuity mistake: After Maurice is thrown out of the tavern Gaston whispers his plan to LeFou, and when they begin to dance we see an angled wall with a dart board, bow and arrows, but the problem is that angled wall with the dart board, bow and arrows did not exist in the overhead shot, right before Maurice walked in.
Continuity mistake: When Belle is reading her book before Gaston walks in to propose, the bellows hangs from a hook on the fireplace stones under the mantle, but when Gaston is talking to Belle the bellows now hangs directly from the wood mantle, though it's back to the original way when Gaston knocks over the rocking chair.
Continuity mistake: During Gaston's song, when they sing "Give five hurrahs, give twelve hip-hips," LeFou's beer tankard switches from his left hand to his right hand, between shots. Additionally, one of the wall's wood beams near the fireplace has vanished behind LeFou, then reappears. The same thing happens again later, after they toss Maurice out the door.
Continuity mistake: During scenes at the tavern, the shelves between the brew barrels keep changing between being quite ornate to being ordinary shelves.
Continuity mistake: While Beast is bathing in the bathtub Lumière is standing on a stool with two level foot rest bars, but in the following shot the stool has only one level of foot rest bars, then it's back to two levels again.
Answer: The Rose started wilting at The Beast's 21st birthday and could have presumably started wilting years before the first time we saw a petal fall off. Given the fact that Lumiere stated that they were living with the curse for 10 years, prince Adam could be 23-24 years old at the time of the original movie,13-14 at the time of the curse, and the Rose could have been wilting for two to three years. This is my best guess given the approximate age of Prince Adam in the ripped up portrait before he was the Beast.