If there is glue involved, extensions cannot be easily removed. But extensions installed with crochet hooking can be removed at any time, given enough patience—regardless of age. Here is JR’s advice for removing extensions—something she has done multiple times before:
Just pick them out. Pinch the sides of the junction of your hair and the extensions and pull and tug until little bits of hair come out. Keep doing this until they’re out. Just pick them apart hair by hair.
It takes a long time, but it is worth it.
Hope this helps! If you have any other questions or need elaboration, feel free to ask!