I agree that mid and high level players have gotten better for the same rating. Look at old casts of mid level games and compare them to now. Maybe decision making isn't better for the rating, but the micro, eco balance, amount of cheese, and the number of units seems to have improved. There are mid level players who I've seen play consistently and their gameplay looks pretty much the same to me but they've dropped in rating because the people around them have improved slightly. At the absolute highest level it seems like there hasn't been as much clear improvement outside of maybe better builds, but that's at a level of play out of reach by the vast majority of us.
This is hardly unique to FAF, I've seen this in practically every old enough game with enough of a player base. The more competitive the game, the more dramatic the change is over the years. Hell even at the highest level I'm sure it's not a stretch to say there's some improvement, but it's hidden by the fact that when these old pros come back and derust they're derusting vs people who improved slightly and they themselves quickly pick up some of those subtle improvements building off of their already fantastic fundamentals.
More on the topic at hand, I am 26. I'd be much better at this game if I took it more seriously in high school/parts of college instead of just chilling with super casual play and lots of survival style games. I had more time and far more energy. It's harder for me to improve now, not so much because of age, but because I've got life to deal with. I've got a tech job that saps my brain's energy and I'm constantly torn between putting energy into getting better at this game and just wanting to play a few games to chill and destress.