How realistic is it to play professionally if I start learing guitar at 21?

never too late? ive figured that at and my current rate of learning (at least 3-4hours a day) that in 2-3 years i would be fairly adept. yet i dont think i've ever heard of many cases where musicians started so old. so just trying to get some feedback from all you musician folk etc. out there. I really believe in my capabilities. many thanks ; )