i’d say from time working in bike shops an alloy hard tail – about 4/5 years.
full Sus (off the shelf frame i.e. specilized epic etc) about 3/4 years
full sus (boutique ie turner, ellsworth, litespeed etc) 5/7 years – they do no matter what ever folk say tend to last longer as bushing/bearings/finish are better and also they tend to not change colour/design/geometry every season.
steel hardtail or steel road – considered ‘old’ about 5+ years – but not considered ‘worn’ as such if originally of good qaulity (generally not included in this is cheaply fabricated stuff) for examply an old rocky mountain blizzard will be considered desirably compared to a similar age steel specilized etc, usually also the geometry of the canadian stuff and also kona’s gives it more value, as it tends to be considered more modern.
it’s pretty tricky – ‘old’ is an odd concept very subjective.