Replacing worn parts before they fail is all part and parcel of maintaining any machine. Knowing when to call time is easier for some parts than others however.
While the chain and sprockets will gradually perform less well as they wear out, parts like the crank arms and handlebars don’t give such clear signals that they are nearing the end of their useful life.
We subject all of our frame parts to stringent testing in-house, and our bikes have to independently assessed against the relevant ISO and EN standards for a City Bike. However, as with all bicycles, aluminium parts such as these will eventually be subject to material fatigue. Aluminium is a great choice for these components for a number of reasons: it’s lightweight, stiff, and cost effective. However, it is also less resilient than steel, and for this reason we recommend aluminium parts, particularly safety critical components, are replaced within 5 years or 5000 miles - sooner for vigorous riders, and especially after an accident or significant impact. These parts include, but are not limited to, the crankset, the pedals, hinge clamp plates and the handlebars.
Full warranty terms can be found here: https://brompton.zendesk.com/hc/en-us/articles/115000554605