I am 17 years old & am just a 12th pass out. Can I join a graphic design college?

Yes, of course! You can try for NID, NIFT, CEED and many more entrance exams.

Yes, Graphic designing doesn't depends upon the age. Minimal school qualification is enough with lot of creative ideas. Thinking out Of Box in designing skill is enough.

Yes. You can join graphic design after 12th. The below gives you an idea about the top institutes providing graphic design courses. https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://www.shiksha.com/animation/graphic-web-design/colleges/colleges-india&ved=0ahUKEwjd0OKS_fzVAhVCO48KHaBjDH