While I'm a few years away from the age number in question (but close enough

), there has unfortunately been a reduction in both firmness and ability to maintain an erection from even a decade ago.
I don't have any difficulty getting erect to either imagery or stimulation, but as others have mentioned, it's more of a matter of keeping it if for some reason I become distracted. In others words, it is becoming quite easy to lose it with a change in stimulation or mental state. Once that happens, I can sometimes get it back right away with proper stimulation, but other times I'll need a certain "cool-down" period in order to continue. Fortunately when it comes to sex, my wife has always been understanding and we will switch to other things like touching or me continuing to manually stimulate her till I get back a full hard-on. Her giving me oral is usually a sure-fire method and sometimes she will just finish me off that way rather than take the chance going back to penetration.
One area where there has not been much change, if any, has been in morning wood. I can still get hard enough to cut diamonds in the morning; the only drawback being I'm rarely in the mood for morning sex and as any guy knows having an erection does not always indicate desire.
Perhaps the biggest change that has occurred over a longer time period has been in the angle. When younger and first experiencing regular erections it was straight up vertical, against my stomach, when standing; while now it is all but horizontal and sticks straight out. While I've never regularly measured it, I swear I've likely lost close to an inch at least over the years, but I'm willing to bet has more to do with also having more middle-tire spread as well.
Overall, it still fortunately serves me well even if the changes (along with the ones in the rest of my body) prove frustrating at times. One just has to accept inevitability I guess.