Congratulate them.
Just because you had a crush on someone once doesn't mean that you need to be jealous of anyone who gets close to that person ever. If you're saying that you still have a crush on the person despite only seeing them rarely, that's not a crush, that's a fantasy, and one you should probably give up.
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This isn't 1950. Girls develop way later than they used to because human lifespans have increased - Lavos_Fanboy