Age 65 here, with the same problem. It helps a LOT to do the following:
(1) increase detail in AH. Maybe resolution too, but just increasing the detail will give you the occasional glint on a metal surface, or some more intense colour, to keep you focused.
(2) Get a monitor with variable settings of brightness for games. I use a 19 inch Viewsonic (2 years old) and push a button on the front of it to set it to ultrabright x3 for gaming. (If you are watching porno flicks, press the button again for 5x bright for real clinical display ROFL). At ultrabright x3 there isn't a bandit that can hide in the weeds.
(3) Go to your local pharmacy, and buy glasses which are little more than magnifying lenses, and use em only for AH. It will sharpen your vision a whole lot on the screen just feet in front of you. Probably anywhere from 1.5x lenses to 2 for starters. You should be able to test em in the store. Put a pair on, and try to read the small print on aspirin labels etc. No fooling... this makes a HUGE difference for me! It also helps with taking the right medication, I might add.
I am serious about numbers 1 through 3. However, you may also find the following useful.
(4) Don't sneeze while you are drooling. It gunks up the screen and looks like terrain if you don't clean it off right away.
(5) Napping while playing AH is not recommended, but sometimes the benefits so outweigh the risks that . . . .
eh?