I have been a single figure golfer since I was a junior, got down to 2 handicap at the age of 35 where I remained for 10 years sometimes down to 1 for a few weeks, in September 2011 I finally reached Scratch and have managed to maintain it since now aged 46! I do not yet feel I have reached by peak in golf and always trying to improve. My short term goal is + whatever I did get to +0.4 last season, my long term goal is European Seniors Tour Qualifying School.
So my answer would be if you are 12 handicap at the moment there is a long way to go to get below 5 never mind scratch or +, if you are dedicated then I think Seniors Tour maybe a goal but an outside one, but hey go for it ! and Enjoy the ride!