Always know your limits. One bad event, and your decision will be made for you whether you like it or not. And then what do you have to offer your students?
Taekwondo is fast becoming a young man's sport. Too often, my daughter, or other women in our dojang, had to accept a trophy after winning by default because there were no other competitors, or the competition was adjusted so that she'd compete in a different category, or that the event was simply cancelled. Besides a waste of money, it's a waste of lots of people's time - not just the tournament, but for the practice leading up to it. For older women, the prospects are even worse. Maybe that does not apply to everyone. But that can factor in one's decision to press on in competition.
Older folks can't - or shouldn't - be doing dangerous flying or twisting kicks; a blown knee, torn ACL, a bruised hip, a twisted ankle, or even a pulled muscle - these can spell disaster for older folks, who are statistically generally well-established in their day jobs and don't heal well from injuries.
If you are well-conditioned and you know the risks, by all means continue if you want.
As for me, I too am approaching an age where young kids can knock my block off - and I'd never know where it came from. Arthritis has set in, and aches and pains take a lot longer to heal than they once did. Competition is just too dangerous for me.
For me, I am an instructor, a boy scout leader and merit badge counselor, a soccer coach, a baseball coach, and a softball coach. I do photography and videography as well. And I volunteer at an Elk's lodge and our town drum corps.
All of that, including my responsibilities at home, comes to a grinding halt if I experience said knee, ankle, or hip injury. So I stopped competing in sparring long ago. I compete in forms and breaking every now and then, but, I define my contribution to my style in terms of what I give back to the community.
As a result, I spend significant time reading up on relevant subjects, or going to seminars, and then passing what I learn on to my classes. In addition, I referee, I help administer gradings, and I assist in setting up tests, demonstrations, and tournaments. I no longer feel the need to compete to prove myself; rather, I give back what I've learned. Not to be (too) cliche, but, the light of a candle does not dim when it provides light to another.