15 year old deck

We have a deck that is about 15 years old that receives about half a day of direct sunshine. We are located in Indiana, so we have all types of weather, including rough winters. The deck has multiple layers of stain that has begun to peel off, flake, and is pretty weathered. Some of the boards have been replaced with new treated deck boards, as well as new wooden steps have been added to the front of the deck. We are looking for a way to bring back the life of the older boards, but also match up with the new boards and steps. What would you suggest? Thanks for your help, as hours of research have been put into trying to find the ultimate solution.