I want to start a new page selling affiliate products. Should I start a new domain or see if I cant find an older domain that fits my niche?
As far as SEO goes, buying an older domain would be better. If you feel you can use a more descriptive/creative domain and be OK with staying in the sandbox for a while, get a new one.
As long as you can optimize it well, it will really not matter. What counts is not that much the age of the domain as it is the age of the file that is been published.
Trapped is right. Don't worry about the age of the domain so much. Instead, work on finding an easy to remember brandable domain that has at least one of the keywords that you want to target.