A lot of good comments. Health can be a major factor, age is just a number, for me its all about your mental state.
The willingness to drive on, the motivation to still compete even with complexities trying to hold you back.
This basically means to compete you have to practice more and train harder.
Lots don't though and that is our demise. ChrisC shoots in NEFTA which is littered with World and European Champions and is very competitive.
I found though that (being based in Teesside) the travelling was a royal pain. Hull for instance and Anston were two hour drives away for us to shoot 30 pellets in a field at targets mounted to scaffolding (terrible) or a very dusty windy quarry.
Someone with Chris's dedication would travel and compete every week and strive to be the best at each event. I remember a junior shooter from Sedgefield who became world Champ would turn up every week to shoot a full FT course, but if it wasn't set up at his club his dad drove him to Redfern's or similar so he could shoot a full course. Dedication. We decided to only compete at 3 or 4 clubs we enjoyed visiting. My only motivation then became enjoyment of the sport and therefore stopped practicing as much and then stopped competing. Life, family and other hobbies take over the time previously dedicated.
At 52 I still shoot well. All the years of practice and knowledge doesn't just disappear. I can still score 18-19 on the sillies and hit 35mm (home made) target, free standing with a Rimfire at 75 yards.