How much does a car's reliability decrease with age and mileage?

I'm considering changing my 2009 vectra for a 2014/15 insignia. A new car is always a nice idea but one of the big factors is reliability. At 70k how likely is my vectra to let me down? It has been regularly serviced. Changing cars is obviously expensive but you get a new car out of it, I'm not a fan of pouring hundreds or even thousands into repairs on an older car. I know there are probably cars more reliable than the insignia but then that's reflected in the price, a 2014 insignia with 20k miles is a similar price to a 2012 Honda Accord with 50k miles (that bit closer to some major repair?)
So what I'm wondering is how much does reliability decrease with the age of a car? Clearly even a new car can break down but at what age does reliability fall off the edge of a cliff? I assume a 1 and 2 year old car would be similarly reliable, how about 5 and 7 year old for example?

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