There will be many cars that have gone way over the milage suggested for changing Timing Belt and Timing Chains.
But that doesn't mean you should leave it, because if the Timing Belt breaks then you will most likely require a new engine, so it's much safer and cheaper in the long run to replace them at given mileages, and even the mileage suggested does not guarantee that the Timing Belt won't break before then.
Always check their condition and if they are looking a bit the worse for wear then get the spanners out !
While you are doing the Timing Belt it is worth checking the Water Pump and why not give your car a full service while you're at it, spark plugs, oil change, change all the filters Including the Cabin/Pollen Filter (these are often forgotten about, but it will help your interior fans and stop a few smells) and even the Ignition Coil Pack or just the spark plug leads and points on old vehicles. Your car will appreciate it by running better and you will appreciate that their will be less likelihood of a breakdown.