I go by the 10 year rule if its a car I plan on keeping for awhile. I don't want to deal with problems, and with an old car you need to be proactive with maintaince in order to do this. My old Camry split the plastic radiator at the 10 year mark, my dads F150 just did at the 13 year mark. (but it has a lot less miles on it then the Toyota did) The SDL's failed at the 18 year mark, go Behr!
At 10 years if the radiator is $200ish or less I'm replacing it, if its super expensive I'll keep an eye on it.