I held out on using a shock collar on my lab (flushing) for 4 years, and then finally got one for her and trained her on it. It was the single greatest improvement in my enjoyment of our hunting together. So from a flushing lab point of view, it's not too late. However, it is perfectly likely that there are things about pointers, or GSPs in particular, that I don't know that might be different. I found it incredibly easy to train her. Because of all my efforts over 4 years to train her, she KNEW every command inside and out (she would just decide to temporarily ignore them when overcome with an intoxicating snoot-full of phesant scent!). I basically turned up the settign until she just barely twitched her head/ears when I hit nicked her (no yelp, no cowering, just "annoyance" that might come with a pinch). I then used the "continuous stimulation" button (not the nick) to train her to "turn off" the collar by obeying QUICKLY, and then eventually, to avoid a stimulation at ALL by obeying IMMEDIATELY. I simply got her attention, then pressed the continuous button (on that same very low setting) a moment before saying the command (in this case, it was whistle sits). As SOON as her rear end hit the ground I released the button. In a single session she was sitting IMMEDIATELY to beat the simulation. I then repeated with the "come whistle" (hit the button, give command, and release button as SOON as she started moving towards me). That wsa pretty much good enough for me. The result of a very few training sessions was that I went from a lab that I was constantly jogging after, and yelling, whistling, doling out milkbones etc etc etc... to a lab who I could stop anywhere, anytime, and have come back to heel, anywhere, anytime. In that first year I might have had to "remind" her wiht the collar a half-dozen times, but now I can't remember the last time I actually put my hand on the controller. Anyhow, for what it's worth, that's been my experience with teaching and old dog new tricks. That said, I've heard of some peoples' dogs responding very negatively to the collar, including my dog's littermate, who is now terrified of ducks because she thinks they are electric (that was a case of an e-collar being put into the wrong hands....).
Good luck
-Croc