I see zero difference in my stamina and I'm 49.
Peronally, I think if you have taken care of yourself all your life training does not have to change much at all as you get older. It's another one of those myths, like "the hard gainer" myth.
I do about 80 to 100 sets per week, plus cardio work. I refuse to concede one inch of groud because of age. In fact, I have set new and more difficult goals to achieve. I'm stronger, faster and most importantly smarter than I was 20 years ago.
Let everyone find their own path, but here is what I do:
1. Eat naturally-Fruits, vegetables and meats. If man makes it don't eat it. (Jack Lalane said that first and he's right, he will also be 91 in September and trains two hours per day).
2. Supplement regularly-I take (natural) vitamins and supplements three times per day every day and have for many years. Just make sure you do the research and take the right ones. Stay away from drugs. I don't even take a Tylonol. Have you noticed that every few years they come up with a list of new side effects for these long time "trusted" drugs?
3. Sleep 8 hours per night. Forget what they tell you relative to everyone being different, get your 8 per night if you want to stay healthy and continue to gain muscle.
4. What goes into the mouth is not as important as what comes out. Be honest with everyone, treat people the way you want to be treated and give to others. It's amaxzing the positive impact this can have on your health.
5. Craft a challenging routine, but don't do it for more than eight weeks. Then take a few days off and raise the bar for the next 8 weeks. Keep your sessions 30 to 60 minutes, and no longer. Actually, I rarely train over 45 minutes. Oh..and stay away from joint killers like the Barbell Bench Press.
6. I'm big on restorative therapy. Swim, massage, heavy use of ice, ultra sound therapy. You name it and I do it! I can always afford an extra 20 minutes per night if it means I get to keep training at this pace.
7. Stay away from drugs. I don't even take a Tylonol. Have you noticed that every few years they come up with a list of new side effects for these long time "trusted" drugs? My wife says I'm a bit fanatical when it comes to this, but I have not been to see a doctor, except for an every other year check up, in almost 20 years!
8. Do your cardio! I don't what the latest guru says about cardio and muscle wasting. Hit cardio two or three days per week. I do one day of long distance and two days of "sprint" training; hills, 1/4 or 220yds, whatever I'm in the mood for.
9. Keep a positive mental outlook regarding your age. Don't ever let anyone tell you that you can't do something because you happen to be 10, 20 or 30 years older than them. They are stereotyping you! No different than someone stereotyping another because of sex, race or any other reason. It's pure BULLCRAP!