I think the most major complicating factor you've not modelled is mileage — that's very nearly as important as age in both determining a car's value and in its longevity and reliability.
To complicate matters, different people drive different amounts. I, for example — and I've not done the maths — reckon it makes sense to buy an older car with lower mileage, since that will weather 30,000 miles a year better than a young high-miler.
Running costs will be hugely variable between cars, based on their fuel economy, reliability, longevity (more precisely: tendency to become less reliable as they age and rack up miles) and original purchase price (a £10,000 secondhand BMW 7-series costs more to run than a £10,000 brand new Ford Ka). Unfortunately, two of those factors are subjective. But they matter: last year I happily bought a car that had done 130,000 miles. For a large-engined luxury saloon with a reputation for impeccable reliability, that makes sense, whereas I've known cars to be completely past it at 100,000 miles.
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To complicate matters, different people drive different amounts. I, for example — and I've not done the maths — reckon it makes sense to buy an older car with lower mileage, since that will weather 30,000 miles a year better than a young high-miler.
Running costs will be hugely variable between cars, based on their fuel economy, reliability, longevity (more precisely: tendency to become less reliable as they age and rack up miles) and original purchase price (a £10,000 secondhand BMW 7-series costs more to run than a £10,000 brand new Ford Ka). Unfortunately, two of those factors are subjective. But they matter: last year I happily bought a car that had done 130,000 miles. For a large-engined luxury saloon with a reputation for impeccable reliability, that makes sense, whereas I've known cars to be completely past it at 100,000 miles.