Age I am not sure about. I sold an elder horse for about 15,000 so I don't imagine it hurts it much if at all because that's about what I get for my adult horses that I let get skilled up and get a bunch of good traits like this one.
Traits and skills make a huge difference. I've sold a horse as cheap as 800, he had no skills and I think his only traits were untrained, fast, and aggressive. Horses with no skills and only a few traits but the only bad one being un-trained have sold as high as 5000 for me.
The tool-tip for untrained says it makes a big deal with the sale price, but in comparison to other factors, I don't think it's as big of a hit, so I find it misleading. Depending how much space you have and what your goals are, this may or may not be something you want to focus on before selling a horse, rather than just selling it faster to breed another.
I get about 10000-15000 for horses with 3-4 good traits and about 5 skill points in each racing and jumping. Part of that I am sure is the bonus from the parent getting the achievement for 15 wins.