Other mistake: There are two gears on the top of Belle's roof, but both of them are spinning counter-clockwise. (00:02:45)
Other mistake: In the dance scene, when Mrs. Potts says "true as it can be", in one frame her eyes turn black and then turn blue again. (01:00:25)
Other mistake: At no point in this movie is the background of a room ever reflected in a mirror - even if the characters themselves are reflected in it.
Other mistake: After Lefou is stabbed in the butt by Cogsworth, there is no hole in his pants. This is impossible, since the scissors did pierce him.
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.
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.