As teenagers, men can generally get an erection in the time it takes to lick a stamp. But as they age, achieving a full, firm erection takes longer and may sometimes require a little manual help from a partner. In a study at the University of Southern California, for instance, researchers showed the same sexy movie to two groups of men, one group aged 1 9 to 30 and the other, 48 to 65. The younger group got erections almost six times faster than the older guys.

With age, there's also a gradual increase in the recovery time between ejaculations (the amount oftime it takes to be able to have a second erection after the first one fades). "An 18-year-old man can climax up to eight times in 24 hours; when they're 35, once every 24 hours is more the norm," says M. D., director of the Graduate Education Program in Human Sexuality at the New York University Medical Center in New York City. There's also a gradual decline in the expulsive pressure and the volume of semen that's expelled during ejaculation. The aging male may notice something else, too: More and more, as his arousal nears climax, he no longer feels the unstoppable urge to ejaculate. Perhaps once out of every three or four times he has intercourse, he simply lets it go without climaxing. Consider it a prerogative of age.

He has fewer morning erections as he gets older, too. And he is likely to have sex less frequently. Dr.reported that married men between 31 and 35 average about two orgasms a week (whether by intercourse or masturbation); men between 56 and 60 average about two a month. As he ages, a man's testosterone levels also begin to dec.line, but very gradually-and that doesn't seem to be what causes his gradual decline in sexual desire. In fact, men who have gotten experimental injections oftestosterone have a few more sexual fantasies, but the hormones don't produce any startling blast of libidinous desire, nor do they enhance the ability to produce an erection. When testosterone levels in an aging man are subnormal, however, injections may prove helpful. (See "Testosterone.")