Hhmm...This is interesting.
As with self-rating any skill, whether work related or hobby, there's always going to be the issue of modesty, ego, perception vs reality, differing standards, and regional bias. Curiously, few have chosen "expert" although some of the fishermen who have posted on this thread would certainly receive that designation from me.
I like Squaretail's thoughts on this.
Obviously, we all have new things to learn and days when we catch the skunk - but this is true for Lefty and Whitlock and the sport's leading practicioners. Even the best at something have strengths and weaknesses. My unabashed opinion is that my biggest strengths are fly tying and casting and my biggest weaknesses are knowledge of hatches and bug ID.
Certainly, time on the water helps and my 30+ years at this game matters in my mind, esp considering that it has been an intense 30 years (some long time FFers never get passed beginner - even though they still enjoy their outings). I fish a lot and fished a lot in my teens/20s, taking the sport seriously. I'm not sure I could out fish today the younger version of myself from the 1980s. I think one can reasonably become an expert in a few intense seasons of FFing, tying, and reading about it if they're really committed. Guys like JayL, Sal, 3wt7X, Ryguyfi and others are impressive in their skill, dedication to this sport, and willingness to help others - they remind me of myself two decades ago.
In the end, I think a big part of what would transition an FFer into the "expert" or borderline-expert.....would be the capability to think beyond the conventional and really intensely focus on new aspects of a FFing angle. I'm on the record as believing that much of what makes one a very good FFer is possesing a keen sense of observation. Some dedicated, long time FFers are perfectly happy to enjoy the sport without this intense focus on details. I suppose folks like this can be categorized as "old hands" but I think to reach "expert" one needs to master real specific detail and develop some unique and unconventional stuff.