Youth certainly is fleeting. A new survey finds that men start to become sexually "invisible" to younger women once they hit age 39. When polled, 62 percent of women said they assume that most men in their late 30s are taken and not worth approaching. Fifty-four percent of all respondents said that the number one sign a man had become "invisible" was that women no longer check him out at the bar. Other signs a man is aging out of the dating game: having a double chin, flecks of grey hair, thinning hair and bad teeth.