It can totally be done!
I got Sydney when she was 6 years old. She did not know how to "sit" or "down" on command at all, she was a breed ring dog and wasnt taught these things.
I started training her for her CD (Companion Dog) Title when she was 7, and before she was 8 she had her CD and RN (Rally Novice) titles. Not only did she get these titles, but for her CD she placed for all 3 of her legs, two 1st places, and one 2nd place. Her highest score was a 194 and her lowest a 189.5. She also ranked nationally for the novice class, in breed and non-sporting. In 2007 she ranked the 4th highest score average for CD titles earned by all Dalmatians that year.
I would encourage you to do the CGC for sure!
FOREVER in my heart:
CH Erin N Shamrock's S Hemisphere CD RN CGC
(1/3 UWP) U-CH Shamrock's Shippin Up To Boston RN CA NJ-N TT CGC "Shane" (4yrs)
"Ocean" Alabama craigslist dump (2yrs)