Sure, you know that you're a better teacher than when you started. But can you prove it? What can you do that a fresh-out-of-college teacher can't? What can you do better? How can you show that you do your job better?
As a programmer, I have a similar problem. But I don't just throw up my hands and say, "Well, some things just can't be quantified." Instead, I try to quantify them. I try to show how my estimation skills have gotten better. I try to show how my code now has fewer bugs and requires less maintenance effort.
As a teacher, what can you show me to distinguish you from a wet-behind-the-ears graduate?
I can plan lessons that include activities tailored for the needs of specific students. Training teachers find this hard and tend to plan for the median.
I can plan lessons more quickly than a training teacher, thus liberating time for more tailoring. I can use technology to differentiate delivery and to save time.
My feedback to each student is more accurately geared to that student's 'zone of proximal development'. I can set targets that mean something to each student.
I can 'reflect in action' in the classroom. I can read the situation in the whole class, and I can understand the logic behind mistakes that individual students make, and suggest alternative approaches.
How can you show that you do your job better?
In the institution where I teach we have QA observations and peer observations in place. That gives me some kind of benchmark.
Education is fuzzy, hard to measure, hard to standardise and quite hard work if you do it properly. As are many human occupations.
I responded to the professional development aspect of Patio11's comment. My point being that seasoned professionals do have wisdom to impart on less experienced colleagues. I used myself as an example but the point made is clear (I think).
Even if I really am not a better teacher now than I was 17 years ago the point is still true. Well, in order for the point not to be true one has to believe that there is no wisdom gained from experience is teaching an that this is universally true. I know of no profession in which practitioners do not generally get better over time. Indeed,there is no profession in which no practitioner gets better over time.
So what is the point of requesting that I prove that I really have gotten better over time? It's not germane to my point unless one believes that no one ever gets better over time in teaching.
Look, my Mother has 2 PHD's in education and often does professional workshops for teachers. However, like most forms of professional development her workshops are both expensive and their value is hard to quantify. So it may pay better than her normal job but as she says it's of limited value.
Programming is a lot like teaching it takes a few years to get up to speed, but vary quickly years of experience stops being a useful metric.