What kind of vitamins should i take at the age of 20?

Maybe Vit D.: If you are able to eat a healthy diet with lots of vegetables of all colors and varieties, whole grains, nuts & seeds( unroasted is best); fresh fruits or frozen if unavailable; and complex carbohydrates, you will get all the vitamins needed except vit b12. That has to be supplemented or eat some animal products etc. Vit. D is best gotten from sun exposure but may be added as vit d3 in capsules.

A general multivit: For most persons a general multivitamin is a good idea. Taking some supplemental vitamin d can also be of benefit.