If you wear glasses, look at getting tri-focals - I consider this cheating - but what do I know?
First off - I'd reccomend taking a class.
Also, practice on this: Extend your finger out in front of you and focus on the tip. Then change your focus on something 10 to 25 yards away (for pistol, 100 or so for rifle) and once you acquire it, change your focus back to your finger tip.
On top of that, dry fire, dry fire, dry fire - just because one aspect of your shooting ability is going to shit doesn't mean you can't still improve in other areas.
Boots Obermeyer, Mike Bykowski, Masaad Ayoob, and Jim Cirillo are all old farts - but I've seen them shoot and know that I wouldn't want to be on the receiving end of what they're dealing out. Old age, for a lucky few, equals wisdom.