Factual error: After Gaston stabs the Beast in the fight sequence, there is no blood shown on the knife. [This is a longstanding Disney animation tradition. The House of Mouse seems to believe that onscreen violence is less traumatic if all evidence of it disappears quickly. Thus, you rarely see blood and when it IS seen it's usually minor, even if the wound isn't minor.] (01:16:15)
Continuity mistake: When the song Be Our Guest ends, in one shot Belle's right arm is on the armrest, but in the next she is suddenly clapping her hands.
Continuity mistake: When Mrs. Potts and Chip go to Belle's bedroom, the small colorful decorative pillows that lie on Belle's bed disappear, then reappear, etc., and the three large pillows change their appearance and position in the following shots as well. (00:30:20 - 00:31:05)
Continuity mistake: When Cogsworth comes into the study to say that Belle isn't coming for dinner, he opens the door inward and doesn't shut it. In the next shot of the doors from the hallway, though, the doors are closed, and the Beast opens both doors outward. They aren't double-swing doors like the kitchen ones. (00:32:30)
Continuity mistake: When we first see Belle at the beginning of the movie, there's a bridge in front of her house, which she crosses over to get to town. But later, after Gaston plots with the insane asylum keeper, there's a wide shot of Belle's house with a horse and buggy waiting in front, and the bridge is gone.
Continuity mistake: While Lumiere is seating Maurice in front of the fire, Cogsworth tumbles down the stairs and his nuts and bolts go flying out on the stairs. The camera cuts to Maurice, then back to Cogsworth on the stairs, but all the nuts and bolts are gone. (00:14:40)
Continuity mistake: When Gaston enters the West Wing with his bow, the painting on the right side of the door is different than it is at all other points in the movie.
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.
Continuity mistake: In the 'Human Again' song, near the end of the song Cogsworth and Lumiere turn on the fountain and the large middle statue sprays out water, but the two statues either side of the middle one are turned off. Then the wardrobe jumps out onto the balcony singing and then it cuts to an ariel shot, and now the two side statues are suddenly spraying water too. (00:58:45)
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.
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.