Can i start gymnastics at 17. even if i havent done it before?

I am 17 years old going to be 18 in march. i want to do gymnastics but my parents wont let me because theyll think ill hurt my sefl and also because i have asthma. i would love to do it. i am healthy and a fast runner and i can do anything but i do have a little fear though? can any one help me??? please. :)