While there are cars that can be reliable if properly maintained when older, viz. Toyotas, I probably wouldn't go past 25 years, as even though a car can be reliable at 30 years of age or more, you will probably have to start replacing all kinds of seals that have degraded by then. Granted, in practice, one doesn't see too many cars of that age for sale in Minnesota, aside from collectors' items, as we use a lot of road salt in the winter, and frames often "rot" away from rust by then as a result.