I talked to my followers about that a while ago !
And everything has an explanation !

• The servants didn’t age, they’re objects, the Prince turned into a beast, which is a living creature, feels pain, tastes food, grows up ! so he’s the only one who got aged, and Chip was already born and 4-6 years old before he turned into an object.
• The stain glass prologue doesn’t say anything, it’s an illustration, you can see it as a 11 year old boy or a 30 year old man, and you can actually SEE the difference between the stain glass in the beginning of the movie and in the end !
• What does his “21st year.” mean if it’s not his 21st birthday ?
• And about the painting, it’s a goof ! Beauty and the Beast is the Disney movie which has the biggest amount of mistakes ! and continuity is one of them ! it’s a mistake, they shouldn’t have make that painting !

Look, the song is very clear, they’ve been 10 years under the curse, and the rose will bloom in his 21st birthday ! if only Disney didn’t add that line ‘10 years we’ve been rusting’ non of this would have brought up !
So everything is explainable, except of the painting that shows a grown up man !