Teens or 18-22 which usually covers it is a massive market in porn, by far and away the biggest. But it is split into lots of sub sections, styles and actions. Selling it is either very easy or impossible. The difference is in the skills of the seller.
A few tips. To those who need to improve there returns in this field. This is a very general advice and there are obvious exceptions and will only scratch the surface. LOL
First and the most important.
It's the biggest market, the most covered and the most competitive. If you don't understand it stay out of it or learn about it fast. The teen sites at the top of the tree are the ones surfers will judge your site against. He pays $30 for Teen Dreams, Sapphic, Alsscan, Met Art, etc. So expects the same value for money when he lands on the next site he's thinking about joining.
If you're delivering hits to a site and getting bad ratios that's as much your fault as it is the site you're sending it to. In fact I can't think of why it's the sites fault. If the site's bad why are you sending traffic to it in the first place? Your fault if you don't know the difference between a good site and a bad one.
Go look at some of the top review sites and see the sites that are at the top. Then work out why they are at the top. The sites are well known and most of them convert well. Maybe not for you but then that's more likely to be your fault than the sites fault. Remember you chose it.
Look and learn, work out what the site has that appeals to the surfer. Look at the image content, text and design. Learn why surfers sign up to these sites. Think as a buyer and not a seller. There are many programs that sell to webmasters first and the surfer second. Dangerous thing to do when the surfer has high expectations and so many choices. The webmaster and the site sells the membership. I'm talking about selling here and not just sending as many clicks as possible and hoping. Can work in other niches, not in teens.
Look at how the site covers the section and style of the Teen niche it's selling to. If it's "sex mad teens couples" does the content on the tour have good content that fulfils that niche? If it does not don't send traffic. If you don't know what that section of the niche is about why are you selling it? Same goes for all sections of any niche.
Lastly when you have a site in mind think long and hard about how "saturated" the site's promotion is. You could be 1 in 10,000 working affiliates and this will make selling the site very tough. Some sites convert better than sites better to it, simply on the numbers of surfers promoting it and the odds the surfer knows it.
Respect your customer.
So simple it amazes me how many have so little respect for their prospective customers and expect him to buy. A lack of respect taints your sales. Sexual arousal stems MAINLY from two things, the need to reproduce and memories.
18-22 years olds reproduce better than 38-42 year olds. It's biological and natural that men get turned on by teens.
We normally hit puberty at 12-14 yrs old, the odds on the first arousal to be someone in the same class as us, is high. This is why teens in class rooms sell. MILF is your mates Mum, Big Tits is a big tit memory and the same for most niches.
This is why teens sell, not because we are paedophiles, it's because sexual arousal is linked to memories and biological. Society has made it a sin. Live with it you're selling porn not bibles.
I'm not talking about abuse, degradation, force or incest here. Sub niches and nothing to do with the teens niche in reality.
Teens don't sell!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!! 
Biggest load of BS I've ever heard. And only believed by the truly stupid. (This is me inviting flamers.)
Look around at the top sites and see what they spend on spots, promotion, content and work out how they spend that money on a niche that does not convert. The truth is "I'm not good enough at teens to convert it."
Let the flaming begin. Ask questions and I will reply.