In the Disney version of Beauty And The Beast, the narrator says that spell has to be broken by the Beast’s 21st year (doesn’t say birthday) and Lumiere says later that the dinnerware hasn’t done anything useful in 10 years. I suspect this detail was overlooked when the script was being finalized, but some suggest that the prince was turned into a beast when he was 11, and that therefore the enchantress was being unfair.
Firstly, the prince is adult-sized in the prologue illustrations.
Secondly, we were more than willing to see Joffrey “Baratheon” of Game Of Thrones suffer and die painfully from poison, and in the books he was only 14 at the time. Yes, Joffrey was a king rather than a prince, but there is no king or queen in Beauty And The Beast, which must mean that the Beast’s country is a principality and that he is therefore the reigning monarch. He is described as “spoiled, selfish, and unkind” even before the enchantress visits, which could very well mean “tyrannical, warmongering, and willing to let his people starve” if it weren’t a Disney movie. The enchantress possibly saved thousands of lives by taking away the prince’s power and turning his subjects into furniture and such so that they couldn’t be a credible army on his behalf.
Thirdly, “As the years passed, he fell into despair and lost all hope” could refer to the moment when the Beast stopped bothering to sit down at the table for a meal and use tools such as plates and cutlery. That moment could well have been 10 years before Belle arrives on the scene.
Fourthly, there’s a precedent for people in fairy tales to stop aging while their land is under a spell. In some versions of Sleeping Beauty, she sleeps for 99 years after her 16th birthday, but she doesn’t exactly seem 115 when her prince wakes her up. In this movie, the little boy teacup Chip was clearly born human, as he has no problem using his legs when the spell is broken. His aging and maturity remained the same for the entire time that the Beast was a beast.
Therefore, the prince in Beauty And The Beast could be any age. Tada!