Is there any problem, if someone at age 20 take one day mens' 50+ advantage?

The chief problem with taking an over-fifty or "mature" men's multivitamin is that it will give you little or no iron. (that is a deliberate omission). You can be sure you get an iron-rich diet, or take a supplement. Or you could take any daily multivitamin/mineral that has enough iron and add One-A-Day Prostate Health, which includes zinc (important), lycopeine, saw palmetto, selenium, and Vitamin E.