Recommendations for helmet replacement intervals vary quite a bit, and aren't based on hard data. Helmets don't go bad simply from age - an unused helmet stored well can retain its effectiveness for decades. Instead, helmets lose their effectiveness from environmental factors, such as taking hard hits, or exposed to harsh chemicals or UV light. So replacement interval recommendations are really based on assumptions of of accumulated mistreatment over time. Here's a couple of articles about helmet replacement intervals:
https://helmets.org/up1505a.htm
https://www.consumerreports.org/bike...t-a5593905789/
As far as CPSC: It is true that aspects of the CPSC standard are more stringent than European standards, which is a bit of a sad state of affairs. When CPSC went about formulating their standards, they looked at the then current standards from independent bodies (ASTM, ANSI, Snell), and they basically settled on the lowest common denominators among the 3 of them, such that the CPSC requirements are roughly the same as the least stringent of the standards. This happened over 20 years ago, and the CPSC standard hasn't been updated since. Which means that the European standards must be even less stringent than the least stringent 20 year old US standard.
Fortunately, there are bodies that are looking at improving helmet safety, such as Virginia Tech, who not only do direct impact testing, but also do testing for rotational forces (which have been implicated in brain injuries). And unlike CPSC, which is is simply a Pass/Fail test, Virginia Tech uses a
rating system in an attempt to gauge which helmets provide more protection.
Another factor to consider regarding CPSC is whether you will take part in any organized cycling events. Many organized cycling events require riders to wear CPSC helmets.