The rose she had offered was truly an enchanted rose, which would bloom until his 21st year. If he could learn to love another, and earn her love in return by the time the last petal fell, then the spell would be broken. If not, he would be doomed to remain a beast for all time.
So... Where does it say that the flower would start wilting on his 21st birthday? It said his 21st year, which could mean his 21st year of being a beast...