Look at all examples here. All range from 16 to 23 years of age.

Use your imagination, go beyond the 30 barrier with figures, morphs, textures, hair, clothes, props.
It' s all empty, open and up for the taker.

*And for your lmatest cousins render?

Why are they in an empty scene?
Wanna get attention? Put them in a beach ball play setting. Get some "life", some movement in the scene.*

Not all are in that age range, Agatha by 3-DArena certainly looks older than 23 and there are a few others.  That said, I am really nit picking here as you are correct that older V4 characters is barren territory at the moment.  I have Agatha and use the character extensively but I would be interested in other older characters and, as it is not mainstream, I would also be willing to pay more.  

This is barren territory because there aren't enough customers that reside here. Same for asking the OP to make characters for Dawn, Roxie and Miki 4. The OP is looking for input so he can make more money... not way less. I'll chime in that the promos need much more work in the lighting, posing and presentation. I'm also going to point out that you should never say that one program renders better than others then present actual renders that contradict your argument. It's all about learning your tool of choice.

Lastly, the clothing. They leave much to be desired when presenting your product to a customer.  To be totally honest, your characters' clothing selections (and tattoo options) represent more of a trailer park variety than sexy, and come off as pedo-pervy when you have the young girls in daisy dukes and cutoffs, with the camera focus aiming directly into their cleavage and crotch area. Delete those items from your runtime. Put them in age-appropriate clothing and cover them up. That's really what your QA testers are telling you. There is really a fine line between sexy and trashy. Switch up your choices with classic choices and hair styles and show that your characters are more versatile... like they going to the library, class or a elegant party.. not just riding in a Ford F-50 with a six pack on a Saturday night. Take a look at how the girls in Rendo's hot list are being presented versus how you are doing it and and add in a few renders based on that to break up your presentation. If you are showing the body shape, make one or two renders showing it, but don't make her practically undressed for all your promos.