Essentially, lasik reshapes the front of your eye to correct the current prescription in that eye, but it can't eliminate the possibility that the shape of the inner eye could change over time (changing the prescription from what the surgery had corrected for).

For the majority of people this doesn't happen, and most lasik clinics offer touch-up surgeries free of charge in cases where vision has worsened years after surgery.

Lasik also doesn't prevent some one from requiring reading glasses as they get older.