I agree that the younger, the better if it is possible financially. Career/school is definitely a consideration but this surgery incurs a major opportunity cost. Whether it is your job, relationships or athletic aspirations, there will be something you have to give up in order to undergo this surgery.
If I had to pick an age range, I would say early to mid 20's as by this time you have completely finished growing for the most part and don't have to worry about growth plates being open etc and you are generally in great physical condition in your 20's and you can even attribute your height gain to a late growth spurt. I am getting it done this year at 25 years old but I would have loved to have gotten it done 2 years ago when I had finished university as I didn't have a full time job and other obligations yet. Being in a long-term relationship at the time prevented me from getting the surgery so it is also a matter of circumstance.
I would wait a couple of years to make sure you have finished growing first and then if you still want the extra height, go for it.