For PCA Club Racing, 2005 has been extended to Jan 2017 (couldn't find anything specific to autocross, so regions must define their own requirements). Personally I've always considered autocrossing to be low enough risk for head injury that most helmets will do (save a bike helmet). But, as Jay pointed out, it's your brain
For wheel-to-wheel I would not go more than 5 years with a helmet, again personal choice, so I'd be buying the latest Snell version available.