I recently watched "Enchanted Christmas" and can totally see by Adam's body language and speech pattern that he probably was 11 on that fateful Christmas Eve.
If you are looking at the stained glass in order to discern the prince's true age, you are looking in the wrong place. Depictions of people in stained glass are typically more symbolic rather than literal about a person's status. Besides, the attire worn by the prince in the stained glass seems to be more reflective of Medieval times rather than 18th century France (when the events of the film take place). I think the likeness of the prince, seen in the stained glass, is more of a symbol of his societal standing (although, for obvious reasons, we can speculate that the prince is loosely referred to as a prince and is not of royal blood; he is more likely the son of a lord or other governing noble) than of his actual appearance.
If you are looking to the likeness of the prince in his torn portrait, I do see cause for confusion because he looks to be nearly the same age in the portrait as he does when he becomes human again (but maybe also slightly younger. Let's say . . . um . . . 17). This could possibly be a simple mistake on the portrait artist's part. There are many portraits in which the sitter is portrayed noticeably older (sometimes younger) than they actually are when the portrait is painted. This may be because the prince wished it (which would give him even more reason for ripping it; he thought he would never be able to live up to the expectations he created for himself when he ordered the portraitist to depict him in such a handsome, noble, striking way) or simply because the portraitist was not perfect in his craft and had a hard time depicting age.
If you are also trying to figure out Chip's age, it is very possible that he is ten at the end of the story. This means that he was born human just before the spell was cast. This explains why he seems so comfortable being a little teacup and never mentions how much he misses being human, unlike the adult servants. In "Enchanted Christmas" Adam and Belle present Chip with a storybook, much like the one Lumiere presented Adam when he was only a year older than Chip. Maybe Adam sees this as a minor right of passage for Chip. Maybe Adam saw more in Lumiere's gift than he let on and this is why he thinks a storybook to be a good present for a pre-teen boy. I think the servants had been trying to calm Adam when he found his parents would be spending yet another Christmas
in Paris instead of home with him. When none of them promised that they would make sure his parents would come home, he became stubborn and refused to show any thanks for the gifts offered to him. He chose to act out as a spoiled brat. However, this is only my opinion; the price's behaviour is up for interpretation.
No matter what age the prince was when he was cursed, the animators did wish him to be 21 at the end of the story. One of the Disney animators described the beast as a normal 21 year old male- unsure of himself, nervous about love, and even with a hint of childish innocence- trapped in a hideous, formidable form.
P.S. I know this makes no difference as to the answer to this posted question, but 'Beauty and the Beast was not released in 1994 (unless you are thinking of the Broadway play). It was released in 1991, one year before I was born (sorry, just thought I'd add some extra trivia in there). So yes, I am the same age as Prince Adam when he is broken free from his curse.