You will come across many adoption agencies that feel that a person should be between the ages of twenty-five and forty in order to raise a child. If you are younger, it may be a good idea to wait a few years since it will ultimately be beneficial to your future new child anyway. And if you are older than 40, you'll still have the opportunity to adopt, you will just be subjected to stricter requirements.
http://www.child-adoption-matters.com/adoption-regulations.html
I was just doing some research for my site and came across this: If you are adopting from Russia you must be 16 years older than the child you are adopting.