I have site on old domain (7 years) with old content. I want to replace it with new site currently with NO content. Topic will be the same and content is user generated. Should I simply replace the old site with new one or should I buy new domain to run new site in relation how Google will take this? I have some fears that Google would penalize new site in some way, now or later.
A new site is a new site. You're starting over again. There doesn't need to be a penalty. You're starting at the very beginning and have to work your way up again.
I would recommend keeping the old site up and running and, where appropriate, link to related content on the new site. That will help the new site get traffic and the links will benefit it SEO-wise.