I think that an increase in self-confidence, and a heightened awareness of my own abilities (and limitations) is one of the greatest "lessons" I learned in my journey to black belt. Having had a very successful high school wrestling career, completed several marathon runs, and had various degrees of success in a multitude of other athletic endeavors, I feel I was already more disciplined, both physically and psychologically, than most "newbies" when I started my TKD training (or perhaps that was attributable to my relative level of chronological maturity - more on that later).
That said, however, I think I now approach activities with a clearer goal in mind, and am much more confident that, if I apply myself (perseverence), that goal is within my grasp. Further, although I've always considered myself a polite person, I think I consciously attempt to treat others with respect more consistently than I may have in the past.
As far as the age thing is concerned, oldies take heart! After a very brief false start a couple decades ago, I started my TKD training in earnest just after I turned 40 (I'll turn 47 next month). Although my flexibility, jumping ability, etc. may not be what they "may have been" had I started at an earlier age, in retrospect I'm amazed at the progress I've made. I was tested by Grand Master Sereff for my III Dan Black Belt last June, and managed to achieve both high test score and best hosin-sul (I also received best hosin-sul awards at my I Dan and II Dan tests). I also competed in the senior lightweight division in the recent Nationals Tournament this year, and placed 1st in sparring, second in breaking, and third in patterns - and missed taking the overall championship by a narrow margin. No secrets here - and no great abundance of natural ability either. Just work very hard , very consistently and focus on achieving your goal(s) and you will succeed!
Also, don't give up if you are faced with a setback like having to overcome an injury. During my tenure as a black belt, I have successfully rehabilitated a reconstructed right ACL (knee), and recovered from a laminectomy/diskectomy to surgically repair a herniated cervical disk (neck). (Note also that these injuries did not result from normal TKD training activities!)
Best of luck to the rest of you "oldies" out there!