Continuity mistake: When Mrs. Potts puts Chip into the cupboard, saying "Into the cupboard with your brothers and sisters", between a wideshot and closeup, more teacups suddenly appear around Chip. (00:35:20)
Continuity mistake: In the kitchen, the stove says "I work and I slave all day long, and for what?" and the stone wall behind him is bare. Then when it cuts to a close-up of him saying "A culinary masterpiece gone to waste," two shelves have suddenly appeared on the wall behind him, with pots, pans and utensils on them! (00:35:30)
Continuity mistake: When Lumiere and Belle go into the dining room before the 'Be our Guest' song, in a wide shot when Lumiere says "Ma chere mademoiselle," you can see by the left wall there is a stone statue of a bird. However when it cuts to the following closeup of him, the statue has gone. (00:36:40)
Continuity mistake: In the "Be Our Guest" number, Mrs. Potts notices a large spot of dirt on her side. Then when the napkins wipe her, you can see the one large spot of dirt had turned into three small spots of dirt. (00:39:05)
Continuity mistake: In 'Be Our Guest', Cogsworth is in the blueberry pie, and has blueberry and pastry over him, yet in the next closeup it's gone. (00:39:10)
Continuity mistake: During Be my guest song the menu suddenly disappears from Belle's hands between shots. (00:39:39)
Continuity mistake: When Belle is trying to get up the stairs to the West Wing, and Cogsworth and Lumiere are trying to stop her, when Belle says "You have a library", you can see that Belle is standing six steps up from the foot of the stairs, and that Lumiere and Cogsworth are standing on the seventh step up from the foot. However when it then cuts, and they all start to climb back down the stairs, Belle is now on the third step up from the foot and Cogsworth and Lumiere are on the fourth. (00:42:05)
Continuity mistake: When Lumiere and Cogsworth are trying to persuade Belle not to go up the stairs to the West Wing, they start suggesting other places to go. When Lumiere says "..the gardens, or the library.." you can see that Cogsworth, Lumiere and Belle are all standing on the same step. Then when it cuts, Belle is standing on the step below Lumiere and Cogsworth. (00:42:05)
Continuity mistake: When Belle goes to the West Wing, from an inside shot, after she opens the door, there is a handle on both of the doors. It then cuts to a different angle and pans backwards, and now there is only a handle on the left door. (00:42:55)
Continuity mistake: When the Beast discovers Belle in the West Wing, he starts yelling at her, and she backs into an old, rotting cupboard that sports many patterns and designs on it. Then when the shot changes the cupboard is suddenly new-looking and in good shape and has no patterns on it at all. (00:44:10)
Continuity mistake: After the wolf attack, it fades to a shot of Belle who is pouring hot water out of Mrs. Potts, into a bowl. In this shot Belle is kneeling on the large white decorative design embroidered on the large red rug, and the little bowl also sits on the design too. Then when it cuts, Belle is suddenly kneeling on the red part of the rug and the bowl sits on the red part too, and the Beasts chair is now sitting atop the large white design. (00:46:50)
Continuity mistake: After the wolf attack, Belle has a bowl of hot water. In the shot where she raises the white cloth and wrings it out, there is a large beige pillar directly behind her. When it cuts to a wideshot of the whole room, the pillar has vanished. (00:46:55)
Continuity mistake: In the scene after the wolf attack, Belle is trying to put a cloth to the Beast's wounds, and they then shout at each other. When the Beast yells "Well you shouldn't have been in the West Wing," you can see that he has three slashes on his arm. Then when Belle puts the cloth on the wounds, there are suddenly four slashes. (00:47:15)
Continuity mistake: When Gaston and Le Fou talk to the Asylum Keeper, in a wideshot Gaston finishes his beer and puts the beermug down. Then in the shot when Gaston says "I've got my heart set on marrying Belle", the mug has vanished. It then returns in the following shot. (00:48:00)
Continuity mistake: After the scene when Gaston is talking to the Asylum-keeper, it then cuts to a wide view of Belle and Maurices house. There is clearly no snow by the wall of the stairs or on top of the water turbine. Then when Gaston and Le Fou search for Maurice and Belle and find the house empty, they leave, and then Gaston throws Le Fou into a large pile of snow by the wall, and there is also a very large pile of snow sitting on the water turbine, which has magically appeared, without it snowing. (00:48:35)
Continuity mistake: When Maurice is getting ready to go back to find Belle, and is pocketing compasses and rolls of paper, he takes a scarf from a drawer, leaving the drawer pulled open, with a red scarf hanging out of it. Then when Gaston and Le Fou enter the house, the drawer is pushed shut with no red scarf hanging out. (00:48:45)
Continuity mistake: When the Beast takes Belle to the library, he opens the doors outwards. Then in the following overhead shot, you can see the doorway clearly, however the doors have vanished. (00:50:20)
Continuity mistake: In the scene where the Beast gives Belle the library, the Beast is wearing a white long-sleeved shirt. When the shot changes to wider view of the Beast and Belle with Lumiere, Cogsworth, Mrs. Potts, and Chip in the foreground, look carefully at the Beast. As he holds hands outwards with Belle, the white sleeves are gone. (00:50:50)
Continuity mistake: When Belle and the Beast eat breakfast, in the scene where the Beast gets covered in porridge, in front of Belle is a decorated glass and a basket of rolls with a napkin over the top. In front of the Beast is just a decorated glass. During shots we see neither one, nor anyone else touch these items. However in the final shot of the whole table, in which Belle and the Beast raise up their bowls, the basket of rolls and the Beasts glass have both disappeared. (00:51:15)
Continuity mistake: Chip the teacup has his chip in the right side of the cup. However in the scene when Belle and the Beast are eating porridge, the Beast slurps his porridge, getting it all over himself, and it then cuts to Mrs. Potts and Chip turning to look at Belle, however in this shot Chip's chip is on the left side of the cup. (00:51:25)
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.