If I'm under age of 18, can I take optimum nutrition opti-men multivitamins?

Why. why would you need them> are you eating a normal diet? do you have a deficiency of some sort that you need them. Before taking even vitamins- you should discuss with your doctor. For example, i see you mentioned mthfr DNA mutation in the list of topic tags. If you have this mutation, folic acid in the vitamin doesnt help you. You would need 5L methylfolate- consult with your doctor first.
Supplements. Taking supplements has more to do with your size and health status than your age. Other factors like whether you need a certain form of Folic Acid and B12 due to genetic SNPs in your MTHFR genes could affect what supplements are okay for you to take , as well. .