I find that there are two countervailing tendencies in sex in old age. (Keep in mind that I am describing long-term marital sex, where a husband and wife have had relations only with each other over many decades.)
First, one of the real delights in old age is that sex can go much longer than when I was a young man. The reason for that is simple: when I was younger I just couldn’t take much stimulation of my penis. Any significant attention to it would lead to a pretty quick ejaculation. Old age diminishes sexual responses, and so the stimulation can go for much, much longer. That is truly a delight.
Second, at the same time (and I don’t know whether this is consistent with other men’s experience), I find that sex in old age is often likely to be shorter – largely because my wife (not me!) seems to prefer it that way most of the time. She doesn’t (usually) want as much extended early foreplay (e.g., kissing and breast play) and just wants to “get right to it” – to the sexual union (though oral foreplay is often welcomed). (She doesn’t say she doesn’t want extensive foreplay, but it is implicit in her responses.) I suppose this isn’t surprising, in the sense that, when you have had relations thousands of time, and when your focus is on the personal relationship more than the physical feelings (as it is with my wife), a more leisurely 30 minutes may not seem all that more desirable than a quicker 20 minutes. (I’ll take the 30 minutes, myself, but I am content to accommodate my wife’s preferences.)
Another aspect of sex in old age: it is surprisingly difficult, even after you are retired, to find the time to schedule marital relations. (This may be especially so if your spouse has a strong preference for doing it at a particular time of the day, e.g., in the morning.) You’d think that with the children out of the house, scheduling our moments of intimacy would be much easier, but that isn’t always the case. And, in old age (like middle age, too, I think), “scheduling” seems to be pretty standard, given that you are not likely to be overcome by sudden passion anywhere near as often, and that life is just so busy.
One thing that remains the same: the great delight a husband can take when his wife, in their relations, transitions from “this-is-something-I-ought-to-do-for-the-husband-I-love” to “yes!” (as in “I want this!”)