No, age itself is not a reason by itself. Motivation and confidence is the key factors, and they are not directly related to age.
And reaction times is definitely not as important as you may think, most people have low enough reaction time to be competitive in CSGO, and pro players don't rely that much on just having the faster reaction time, it's much more about setting yourself up for a advantageous encounter and thus minimizing the impact of reaction time, if not you're doing it wrong.
What sets pro players apart from normal players much more is hand eye coordination, they are just much faster and consistent in getting the reticule on target accurately after they have reacted to the threat.
We all know Guardian has the skill for top level play, but he's lost the motivation and confidence to play well. If he wants to stay in the top level he needs to figure out what his problem is and do something about it.