Here's an equation to consider. Two things age a vehicle - years and miles. How gracefully it ages from either is based on the owners. A 2002 with 172,000 miles has been driven way beyond the average 12-15,000 miles per year. But, whoever owned it, may have depended on it and spent the cash to maintain it.
There's also an 'unspoken' idea that vehicles tend to be creatures of habit. If they were driven a lot, they need to be driven a lot, or problems could manifest.
For rough numbers, the average automatic transmission life span is around 150-200,000 miles, engines and differentials tend to last to 250-300,000 miles. Those would be the 'big ticket' items, and strictly averages. On virtually every vehicle, everything can be fixed, so it comes down to how much per month is the repair costs, and how much you like the truck.