Jumping genres for a moment...
A decade ago, in my early/mid 20s (while I was a post-grad student), I was a fairly high level Counter-Strike player. Not one of the greats, but certainly good enough to pull my weight in a team which managed to take home the occasional bit of prize money in tournaments. However, three things happened which meant that I moved on from that phase.
First, I finished studying and got a job. While the hours I was working were probably only slightly longer than the hours I'd been studying (postgrad can be harsh), I now had much less flexibility over which specific hours I worked. I also had a commute that ate up another couple of hours every day.
Second, I started to really dislike the online gaming scene. I got tired of the foul-mouthed kiddies on the public servers and the up-their-own-backside sponsor-obsessed "pro" players. As well as being a player, I was also a league admin and organiser, so I spent a lot of time dealing with this and the bigger "pro" gaming got, the more toxic the high end community got.
But most importantly for the subject at hand, I realised that I'd hit a plateau in terms of how well I was able to play the game. My aim and reactions were probably good enough to allow me to progress further. Not to the very top tiers, but certainly to a higher level than I was playing at. But my judgement and temperament weren't suited for it and resulted in a lot of mistakes of the kind that you can't afford at that level of play. So while I never went cold turkey, over the 6 months after starting a new job, I basically scaled down from being a hardcore competitive player to being an occasional dabbler in public servers. And then over the next few years, I basically gave up on competitive multiplayer entirely (continuing to play a lot of singleplayer and co-op games).
And then, last summer, for a brief window, I got into the Counter-Strike re-release.
Somewhat to my surprise, I was still very good at the game. However, when I recorded some replays and then went back and watched them, it was clear that in my mid-30s, I was good at it in a very different way to how I'd been a decade earlier. My aim was still ok, but my reactions were lethargic compared to how they'd been in the past. I had, however, gotten a lot more patient and a lot sneakier. The kiddies hopping around the levels at full speed could not doubt have picked me apart in a face to face fight, but I was making sure they never got the chance.
So yeah... I suspect that as one set of skills fades with age, some players will develop other traits and skills that offset that. A decline in clicks-per-minute with no corresponding decline in match results in Starctaft 2 would seem to fit that pattern.