@Sarg0n Have you ever read Thomas Kuhn's "The Structure of Scientific Revolutions"? It has received its share of criticism since first being published in 1962, but if you are familiar with it, perhaps the obstacle to your forward progress as an adult is not a lack of skill or dedicated practice but rather the unstated assumptions that govern your understanding of the game and the training and playing decisions you make as a consequence thereof.
A simple example would be that of the beginner who understands the rules and basic mechanics of play but never sheds his "material calculus" paradigm for the "quick and harmonious development of pieces + King safety in the opening" paradigm and is thus forever stuck in the lowest ranks, despite a love of the game.
I have always had the sneaky feeling that older players get mired in their levels because the conceptual glasses that have served them so well are now a hindrance and must be replaced by new ones.
That said, I am an old fart who just recently returned to the game after a 20-year hiatus and never studied the game before 6-7 months ago. But when I did start studying, I was simply dumbstruck at how much importance an untold number of below expert-level players seem to place on the study of openings. Dumbstruck because the most cursory analysis of the data at hand unequivocally demonstrates that the advantage fluctuates wildly in lower levels of play and games are generally lost, not won, because of the ubiquitous nature of game-changing egregious mistakes and blunders. Some may say, "the study of opening theory is necessary to achieve a positional advantage which will lead to tactical opportunities" but practice shows that lower-rated players (like myself) lack impulse control and the ability to recognize even very basic tactical opportunities.
Getting back to higher-levels, this of course is quite different. The advantage varies much less within a single game and the incremental advantage accrued or lost as a consequence of opening theory can often be a deciding factor in a game. But I would argue if one is very strong tactically, one must go very far up before this is really the case, generally. A young friend of mine who recently obtained his FM title attributed his recent 500 point rise in rating to study of tactics, almost exclusively. He told me he never really bothered with opening theory before breaking 2000 FIDE. I have heard this many times from very strong players.
Closing the circle of my somewhat long-winded argument, I would be inclined to believe higher-rated older players stuck at a plateau suffer from a "fear of losing rating points" which hinders their ability to "explore and experiment with a curious, engaged and open mind". The struggle upwards to an elite and envied level was long and hard, and a player's identity and self-worth closely "bound to it", making the thought of losing hallowed ground without any guarantee of success a perhaps unbearable prospect effectively preventing the player from any further significant progress. I'm quite certain this also happens with younger, gifted players. They rise to an elite level but then lose the ability to "explore without fear of rating-point loss" and stagnate for the rest of their chess playing lives.
