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 But when Gaston leads the villagers to the castle later in the film, the lion's head has been replaced by a green shield.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.
Continuity mistake: When the first song ends, Belle has left the village behind and all the crowd watches her ten meters away. In the close-up and the following scenes with Gaston, she's now very close to the crowd plus a baker with a wheelbarrow has suddenly appeared.
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: When Maurice is tossed out of the tavern we get a nice view of the front with its large hanging sign, but after Gaston whispers his plan to LeFou once again we see the front, which is quite different. The sign is gone, and now there are horns hanging on the awning, etc.
Continuity mistake: When Belle opens the door for Gaston there are six steps to the landing leading upstairs, but when Gaston leaves there are only three steps to the landing.
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: When Maurice wakes up and hugs Belle, in this wideshot we see the bag at the foot of the bed and the flap faces Belle, but just before the bag falls over the flap faces the opposite direction, toward the footboard.
Continuity mistake: When Chip fires up Maurice's invention and rides toward the cellar, in the shot facing the cottage we see there's nothing in the cellar handles, but in the closeup as Maurice's machine hacks away at the cellar doors there's a long branch inserted in the handles preventing the doors from opening.
Continuity mistake: After being stabbed with the arrow, Beast falls over a balcony and then falls over on another rounded balcony, next to the edge. Then the angle changes and Beast is lying on the middle of a long rectangular structure.
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.