The beast and transformed servants also don’t age. Chip is still a child when Belle arrives at the castle at the end of the movie, but he was also a child the night the curse was placed.
The whole village seem to think that Belle and her father are strange, but could this be because they actually age? It is suggested that they moved to the village when Belle was a baby and after the curse had taken effect.
This would mean that the curse could have been taking place over roughly 18 years.
When the curse is broken at the end of the film, none of the servants are younger than the people they left behind in the village. The theorist suggests that they would have been much older than the servants.
The theory might not be what’s intended, but it does make some sense!