I would actually trust a 15 or 20 year old truck to last longer than the newer ones, particularly if talking about a diesel engine. But, even the newer trucks should easily go 300,000 miles.
I bought my 2005 Chevy Duramax new and it now has 279,000 trouble free miles on it. The engine has never been apart, original turbo, injectors, etc. The transmission has never been apart either. I've had to replace the alternator, thermostats, and some injector lines; nothing major.
My 2009 2500 Suburban I bought with 99,000 miles on it. I've only put about 22,000 on it but it hasn't given me any major problems: a broken shift cable and a broken exhaust manifold bolt. Repairing both cost less than $500.
