Bob Dole, Rafael Palmeiro, and commercials featuring smiling gray-hairs cruising around in mid-life-crisis convertibles have pretty much put the "old guy" stamp on erectile dysfunction (ED). But it isn't just older guys having problems getting it up. A new study found that one in four men seeking medical help for erectile dysfunction is under 40, and nearly half of young guys with ED have a severe form. These findings show that sexual-performance problems are far more common among the under-40 set than doctors previously thought.

But here's what's really interesting: The younger men in this study had lower body-mass indexes, higher testosterone levels, and fewer incidences of other medical conditions than the older guys. In theory, these health markers should make them less likely to suffer from ED – but that wasn't the case.

"Clearly, from this study, young men had far fewer underlying conditions that are typically associated with ED, so it's likely that other factors played a role here," says Dr. Paul Turek, an expert in men's reproductive health and founder of the Turek Clinic in San Francisco. One possible explanation is that the under-40 men were more likely to smoke cigarettes and use drugs than their elder counterparts. "These toxic habits can affect hormone balance and blood vessel integrity, similar to how they increase cardiovascular risk," which can contribute to ED, Turek explains. He also says stress from jobs or relationships, neurological issues like depression, certain medications, and diabetes and other metabolic conditions that impact blood vessel function can make you more prone to dysfunction down there. But that also means your ED might be a sign of something underlying and more serious, Turek says.

As for the greater severity of ED among the younger men in this study, the doc chalks it up – at least partially – to perception. "How a man under 40 defines 'severe' is likely different than an older man's definition," Turek says. "Sexual performance demands on the body may be much higher for younger than older men, so they might worry more if they can't perform. Severe ED could mean a single missed erection for a young man, whereas an older man might consider missing erections at least half the time to be severe."

The occasional tepid to nonexistent erection is normal, but if you literally can't get it up, or sustain one for long once you do, then there could be a problem. "If you can no longer get a normal, sustained erection, either with a partner, by yourself, or with the usual visual imagery, then go see a doctor," Turek says. "But if there are times when normal erections are achievable – full-on hard ones that last – then you're likely dealing with situational ED due to stress. Of course, that stress should be managed or eliminated, but the chance of a serious metabolic risk is much, much lower in this situation."