I am 21 years old. What will happen if I take vitamin B12 and omega 3 supplements everyday at the prescribed amount? What benefits?

Nothing. At your age, unless you have specific medical conditions that require those items, the supplements would be of no use at all.
Not much. If you have a normal diet, the excess of anything usually becomes waste and stool excretion follows. If you have low B12 or high cholesterol, then these may be helpful for known conditions.

Related Questions

I'm 21 years old what benefits will taking vitamin B12 pills and fish oil pills do for me? I need more energy. What supplements can I take? Thanks.

Living Life. I know of no physician that would recommend vitamins or supplements to a healthy 21 yo male. Energy comes from being active, getting good sleep unencumbered by alcohol or other drugs. The most accurate advice would also to be living a life that includes very little sugars and regular cardiovascular exercise. One should not assume that pills will give you good health. Good health activities do. Read more...