This is because most 30+ year old CS players retired years ago back when there wasn't enough money in the scene to make a living unless you were a f0rest or a NEO. And the ones who have been around the whole time are so exhausted by the game that it's hard for them to maintain form. f0rest is the most notable exception because he's notorious for not really training hard and relying purely on skill. NEO is the opposite. He has always relied more on hard work and outsmarting opponents rather than skill.
There isn't actually a biological reason why someone between the age of 30-40 should be any worse than someone between the age of 20-30 unless you're living an extremely unhealthy lifestyle that will result in an early death.
Roger Federer won Wimbledon at the age of 36 and tennis requires even greater reflexes and reaction time than CS. When he eventually retires it will be because of general fitness problems, injuries, and an inability to move around the court as fast as he used to for an entire match. It won't be because of reflexes... which shouldn't really degrade until he's in his 50s.
Now considering that CS players don't have to be physically active like tennis players, in theory they should be able to compete for much longer. The greatest obstacle as I said before is burnout and loss of motivation from playing the game too much.