First off, let me tell you that struts/shocks are just like brake components-- they wear over time.
Trouble is, unlike other mechanical failures which may happen in the most dramatic ways possible/preceivable, struts/shocks do not fail instantly but do so ever so gently over time. This can be over the course of 10yrs, or 100,000kms or so. As they gradually wear out, they starting to loose control over port holes, panic braking when rain/snow, or emergency maneouvres during highway speeds... and when that happens (when you need your suspension the most), it would be too late.
Me? I can usually tell when the shocks/struts lost 1/2 of their life for I live in a region where soggy rains with oil slicks on the surface of the road is a norm. Compounded with winter chills and snow/sleet sometimes, you can pretty much certain (or be able to tell with relative ease) that your struts/shocks are not doing their job.
I typically repace shocks/struts on all my cars around 6~10 yrs, or between 80,000kms to 140,000kms typical. At that rate, they are pretty much "tired" and worn beyond hopes of handling our road with confidence, and a new set of replacements would restore safety to all my cars to the point where I do not have to worry about them when I need them.
(*just replaced my dad's 7th gen civic front struts with KYB Excel-Gs during last week: original fronts have 140,000kms on them and they are obviously tired and cannot negotiate turns with confidence*)
Q.