No it isn't! I've pointed out many times it was Network Rail who came up with ut and apparently it was considered industry standard long before that. If you can't be bothered to looked at official documents then you're not in a position to determine what is a fact and what is an opinion.
But that doesn't mean that just because something is of a certain age it needs or should be replaced and rightly it shouldn't. If it (a DMU in this case) does its job then NR, TOCs, ROSCOs and the DfT have no problem in it continuing to do so.
Trains are not toys or fashion items, they are high-cost public transport vehicles and each type of train has different issues as regards functionality, reliability, running costs and customer acceptance. That last item usually sits below the other two, except where the service makes an absolute profit, when it may be desireable to court customers with newer stock.
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Of course, there may well be a design life. This is the intended life, at the design stage, up to which it will be economically sensible to operate. Beyond that point a very hard-headed view will likely be required about whether it is more cost-effective to extend the life or replace. It is almost always possible to keep something operating (unless an irreplacable component breaks) but whether it is economically sensible to do so is a rather different issue....
I agree with you, that design life is determined during design. It is used by planners and bean counters to establish whether it is cost effective before acquisition. After the product has been manufactured and delivered, the factors that determine the service life of a train include:
reliability (including availability)
serviceability (maintenance and spares availability)
suitability (speed/acceleration/capacity/loading gauge/length/image*)
regulatory compliance (environmental/accessibility/H&S)
Each of the DMUs being discussed on this thread has a different mix of the attributes above, of which some are more significant on different deployments. E.g. a Pacer may be quite appropriate for a low density branch line but not for a heavily used interurbal route. Similarly, as has been mentioned several times here, 156s are not suited to services with frequent stops, but 150s are. When there are no more requirements (anywhere) for trains with a particular attribute set, then it may be time to retire that stock or at least those that don't have a role. Whether the trains were older or younger than some magic age determined by the designer during the design's creation is much less of a consideration. The cost of replacing trains that are still doing the job, or storing those that don't having any use is the driver of the decision to dispose of them.