Blanket rule has advantages of keeping things simple and predictable
I'm sure some of the old Snell 2010 helmets are more protective than the new Snell 2015 anyway. Think the club just needs to ensure no new safety measures are overlooked- and from what I've read there aren't any major new changes. It seems it's just something that gets changed on a 5 yearly basis.
http://www.smf.org/standards/m/2015/M2015FinalCover.pdf
^^^Explanation on the difference between snell 2010 and snell 2015. Only the testing method has changed, the standards remain the same.
Also from the snell website:
Why should you replace your helmet every five years?
The five-year replacement recommendation is based on a consensus by both helmet manufacturers and the Snell Foundation. Glues, resins and other materials used in helmet production can affect liner materials. Hair oils, body fluids and cosmetics, as well as normal "wear and tear" all contribute to helmet degradation. Petroleum based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy.
Difference between snell V DOT V ECE:
http://motorcycle-central.com/motorcycle-helmet-standards-dot-snell-ece/