Finding someone who's interested in monogamy may actually be more of a challenge in dating than your age is, because poly people are so heavily concentrated in the goth scene and other alternative subcultures
I've been happily involved in the goth/industrial scene for about 20 years now. I know many other goth/rivethead types in their 30s and 40s, and a few in their 50s. Jillian Venters, author of Gothic Charm School, is in her early 40s and going strong - and brilliantly so, if you ask me. Jared Louche of Chemlab makes no secret of the fact that he's 50.
So you needn't worry about your age. Pay no mind to people who think your interest in things spooky and morbid is little more than an extended adolescent phase, and people who think you iphone gay hookup apps won't be able to date within the goth/industrial subculture just because you're in your 40s.