It depends how you age. It's contingent upon genetics and lifestyle choices. Some guys can be in great shape, full of energy and sharp as a tack in their late 60s, others can be totally washed up at 30.
Poker players tend to be bums. Spending your life in front of a computer screen eating pizza and drinking aspartame laden soft drinks is a recipe for physical and mental degeneration. Add to that the unfortunate fact that a high proportion of grinders tend to be on psychotropic medication of some kind and it all starts to make sense. Finally factor in alcohol and recreational drugs, and voila - we have our diagnosis.
If the above profile fits you, fear not. All is not lost. Hit the gym, eat paleo, cut out drugs and alcohol, read 2 books a week, challenge yourself mentally with new skills and projects. Cut off your wifi. No more procrastinating on the internet or fapping over porn. Soon enough the brain fog will lift, testosterone levels will rise and energy levels will be restored.