LASIK at 21. Is it too young?

It is not too early as long as you had a stable vision in the past two years. And a thorough evaluation 

The FDA has approved all excimer lasers used for LASIK to be OK for patients 21 and over (some, even 18). More importantly, it is essential to look at each individual person and how stable their vision is going into surgery. If your prescription has not changed much, you are perfectly fine to consider LASIK at 21. There are some people in their 30s whose vision is still changing, who should not consider LASIK… and there are some 19 year olds whose vision is perfectly stable and make excellent candidates.

you can undergo Lasik, provided you fulfill all the criteria for Lasik. Any one above 18 years of age can undergo Lasik provided their number is stable from past one year. 

As long as your prescription is stable and your evaluation is appropriate Lasik surgery is a reasonable option for you. There is always the chance of mild regression over time but there is good evidence that if collagen cross linking is added at the time of Lasik surgery (Lasik XTRa) that the chances of this can be dramatically reduced. 

21 is not too young for Lasik however the most important issue at this age is that the prescription can still be changing. You want to make sure that your prescription has been stable for the past year. Most surgeons like to see less than a 1/2 diopter change in the sphere or cylinder. Personally I like to see less than this amount in the last 2 years in someone your age. Your doctor should be able to determine your stability especially if you take with you your glasses or contact lens prescription that is at least one year old.