Helmets have a shelf life, and most people say you should replace them every 5 years, or so, although I'm sure many people use them longer.
I think we're up to M95, but I can't remember. If you only plan to autocross, or if you're just getting started, just go with the M rating, since the SA ratings will run as much as 3 times more. In fact, your local club may have loaners, so you don't really have to show up on day one with your shiny new $600 Simpson.
If you don't know how to contact your local club, go to
the Autocross.com club list to find a chapter near you. Call them and ask what they will accept, and whether they have the loaners.
