Past infringement doesn't matter unless the infringer has received notice from the patent holder or if the product being infringed is marked as patented or patent pending. In this case, the first scenario applies--notice is required. From that point on, potential damages accrue.

Regardless of whether the patent has expired, a six year lookback is all that is available. Sue early, you get the look back and future royalties. Wait until later and you lose any damages for infringement that goes back further than six years, but you continue to possibly get future royalties for whatever term is left of the patent.

Think of it as a six year window that floats slowly upward with the term of the patent. As the patent terms nears the end, the window reaches the top and then starts to shut. Six years after the term of the patent, the window completely shuts.