More than age, refractive stability guides when LASIK is appropriate. The minimum age for LASIK is generally considered to be 18 years, although there are exceptions for special circumstances. Most people have vision changes until at least 18 years of age, and some until their mid 20's. If there has been no change in prescription for at least a year in glasses or contacts, then LASIK is a consideration. MOst FDA approved lasers state that the minimum age approved is 18 years which relates to the people who were studied during the approval process.
Some doctors will purposely overcorrect the younger near sighted patient to give them some "room to grow" in that if their vision was to progress slightly more nearsighted, a very low amount of farsightedness will maintain good vision even with this change. For farsighted patients that are young, it is especially important to have a good cycloplegic or dilated eye exam to be sure to unmask all of the farsightedness.
There is also a condition where the amount of nearsightedness does not stabilize, called progressive myopia, and these patients will become nearsighted again at some point after LASIK. Usually these are the more nearsighted patients to begin with but it is sometimes hard to detect who will continue to change.
In summary, it is not so much a matter of age, but a matter of when the vision is stable and this is typically in the early to mid 20's but sometimes sooner. Information from past eye exams can be very helpful in making this determination.