Aparo  1972 vs. 1983

This is a difficult thing to confront as art can be so subjective. i don't think it is as simple as better versus worse as it is more about progression/growth/evolution and change. I think most artists are like athletes: in the youth and prime of age when learning the craft and drawing a monthly regularly (or in olden days doing multiple series) their art is always fresh and evolving in the repetitiveness of producing on a daily or weekly basis. As they age and drawing less they settle into a style or pattern that may not closely resemble their earlier art endeavors. While we read and read and re-read their older art in a comic book we see less of their current art so in our mind's eye we perceive the earlier work as being "better" than the current when the actuality is probably their skills have grown beyond what we "liked/appreciated" about the early artist. 

Think of it as in the beginning they learn to quickly find short cuts and easier ways to present or define their art and as they learn and grow older the skills are finely tuned and then adjusted over time as the artist learns to define their art differently and visually. Many artists are influenced by other artists and begin with copying/mimic/design as their own skills are developing until their own style adapts or develops and the act of actually drawing pushes them forward in their skill sets. 

Every artist that creates in whatever medium whether art, music, acting and so forth must continue to push themselves in finding "more" in what they are doing. Without that drive and ambition they would quickly stagnate and lose interest in what they enjoy. Who wants to remain eternally the same without any change? That path eventually leads only to boredom...

Bryne 1980 vs 2011

We could dig up examples of artists we think improved with age, or vice-versa, whilst others would disagree and dig up different examples. It is all subjective, so no, there is not one definitive answer.

There's an arc here I think. Almost all artists get better with age, at least up to a point; then most decline eventually for one reason or another. Then some get lazy start taking shortcuts (Byrne), others have physical issues resulting from age (Kirby), and some just "decline" because their style evolves into something ugly which they might still consider an evolution (I'd put Carmine Infantino in this category).

Others, on the other hand, just constantly get better with age, like Joe Kubert, who was putting out incredible material right up until his death. But I think those are the exception rather than the rule.

Still, in most of the examples here, we're looking at someone in his prime vs. someone 30 years later in advanced age. If we were to compare art from someone at age 20 vs age 45, though, I think we'd probably see improvement almost across the board.

Just from the examples given; I think Adams fared for the worse, Jim Starlin still going strong, Aparo and Kane just as good if not better, and Kirby and Byrne not much of a fan at any point. But as mentioned with Mr Kirby's eyesight problems later in life, the decline in detail is understandable.