1 calender year ago i was a 21 handicap, with a few lessons and a LOT of practise im now down to 12.0, the other day i shot a personal best +4 at my local course(not in a medal)
id say with the right coach and correct mental attitude and commitment its possible.
Great progress. However I think getting to single figures would be possible but the gap between 5 and scratch is almost as big as the gap from 21 to 12 as it is harder to shave those extra 0.1's everytime the lower you go