There are circumstances in which not having your period for an extended time could be considered "normal." Some medications can throw your cycle off, an illness, suddenly starting intense physical training, stress. But if you are feeling OK, no major changes in your life, and your periods are settled, which they would be at age 21, then you should see your OB/GYN for an evaluation.
It is probably something very simple, but it's just so much better to be safe.
Peace
Greenlydia
I had regular cycles all the way up until January of this year. Went to the doctor and she put me on medication for 10 days the medicine is supposed to reset your biological clock and put your cycles back on normal track. I just finished my last pill And have gotten my cycle back on track. The only downside is I'm having a cycle for 6 days making up for 6 months.