Continuity mistake: When Maurice enters the castle, the front doors have four planks each. For the rest of the film, they have five planks each.
Continuity mistake: The width of the grey rings on Maurice's lantern changes in the scene where he is lost in the forest with Phillipe.
Continuity mistake: When Belle is reading her book by the table, a shelf over the fireplace is visible below another shelf to the left of it. Moments later, after Gaston has entered and Belle asks "What do you know about my dreams?" this shelf is above the shelf to the left of it.
Continuity mistake: After Belle sees the painting in the West Wing, there is a shot of the table where the enchanted rose is. In this shot, the magic mirror is not on the table. In the next shot, it is. Then when the Beast yells "Get out." it is gone again.
Continuity mistake: When Belle finds Maurice locked in the dungeons, the bars change from black to brown in different shots.
Continuity mistake: When Belle comes to the West Wing, she approaches a cracked up mirror. Then in the shot where she is shown right in front of the mirror, the crack on the top right is much wider, and many cracks are differed etc.
Continuity mistake: When Mrs. Potts and Chip first come into Belle's room at the castle, Chip spills tea onto the carpet. In the next shot, the tea is gone.
Continuity mistake: At the start, the Beast tears a painting and makes ten equal sized gashes. Later, Belle is exploring and sees this painting; there are now nine little gashes and one huge gash.
Continuity mistake: When Belle enters the bookstore, the windowsill is red. When she comes out of it, it is white.
Continuity mistake: Gaston's socks are red when he is in the mud. When he walks away, we briefly see them as being orange.
Continuity mistake: When Maurice arrives at the castle and loses his hat, the bricks the hat is over change between shots.
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 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 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: When Belle is walking home with her book and goes up a wheelbarrow, the sacks of flour on the wheelbarrow move closer together between shots.
Continuity mistake: During the song "Gaston", Lefou throws a lot of eggs in the air, but very few come back down.
Continuity mistake: When Gaston and Lefou are talking to the man in the bar, Gaston throws the bag of gold to the lantern's right. However, when this man says, "So you want me to throw her father into the asylum?", the bag is now on the lantern's left.
Continuity mistake: When Belle is coming to the West Wing there is a big statue of a monster with its mouth open very near the door. When Belle is shown running out of the West Wing later, this statue has disappeared.
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.