Do you think the canon informations we have about the Prince/Beast's age and the duration of his curse are convincing? Because on the portrait that Belle finds at the West Wing of the castle suggests his a late teenager/young adult at best for me. But the narrator says the rose will blossom until the 21st year and Lumière says they have been "rusting for 10 years". Don't know if it is a plot hole or I'm missing something.
All I can instantly say for sure is that this is one of the most controversial topics and apparent plot holes about Beauty and the Beast!
Like you said, Lumière states, “Ten years, we’ve been rusting”, which makes it sound as if the spell that had changed them all had been active for that length of time. This, along with the narrator’s statement that the rose would gradually wilt by the Beast’s 21st year (implying it means when he turns 21), would imply that the prince was cursed at age 11.
But like you also say, the shredded portrait of him in the West Wing, as well as his appearance in the flashback in The Enchanted Christmas (even though I said previously that I don’t consider this and other DisneyToon “sequels” as official ones), suggest the prince was likely closer to the age of a teenager or young adult when he met the Enchantress. With this in mind, “ten years” is possibly a metaphor for how long it has been since the castle has had company, not an actual statement of how long the curse has been active.
According to the Disney Wiki page about the Beast, the original screenplay of the movie written by Linda Woolverton actually had the prologue mention that the prince was 11 years of age when he was cursed and that the curse was active for ten years. The screenplay also mentions that the prince was the reigning monarch of his kingdom, while Lumière and Cogsworth (who were unnamed at that point) were his regents. However, this was an early draft, so it is unclear if this concept made it into the final product.
There have been times that I’ve questioned this topic when watching the film agin and became frustrated when I couldn’t think of something valid to prove it. But I don’t dwell on it so much now and just accept it without trying to argue anything, except it does seem like a MAJOR plot hole on the filmmakers’ parts. I guess the best I can explain about it is, I believe the prince was 11 years old when he was cursed, and was so for ten years until he turned 21, which would match Lumière’s statement.
But when it comes to the portrait, I have a concept that it could be a painting of him of what he would look like when he was older, with some inspirations being his physical features as an 11-year-old. That may sound like a weak argument since children’s faces can change significantly from when they are preteens to when they become teenagers. But since it offers a possibility, I’d like to stick with this concept.