Sex drive and Stamina are two different things. My husband is in his 50s. His drive has dropped slightly, although he is still interested in sex a lot. We have sex at least 6 to 12 times a week!
As for stamina, he actually lasts longer than when he was younger. But, he gets tired more easily. If that makes any sense. I know sometimes he'll quit after I have had my orgasm(s) and some penetration play. Even if he hasn't had his orgasm. Then we'll do it again in the morning when he's more fresh. He does have a harder time keeping an erection. Not all the time, but more than when he was 20 years younger.
But, drive (the desire to have sex) and stamina (the ability to have energy and last a while) are completely different spheres of action and feeling.