The oldest cars - Grande Puntos - are now nearer 4 than 3 years old, and the earliest 500s will be about to celebrate their third birthday. Some of them belong to Abarthisti, but form what I recall of various posts on here, no-one has yet exceeded 50,000 miles (some are getting very close), so there is no actual real life data to answer your question, but as Corsa says, provided you look after the engine (keep oil in it, for instance), I doubt there should be a major problem. It tends to be ancilliaries that cause more of the grief in just about any car these days.