Some considerations--the ethanol lobby has been hard at work the last few years. Most fuel now contains 10% ethanol. Why is this important? Bear with me. Ethanol is a form of alcohol, and it is hygroscopic, meaning it absorbs water.
What does this mean for the fuel sitting in your mower that you never use? Well, all air has a humidity level, which is, yes, you guessed it, moisture in the air. The alcohol in modern fuel absorbs the humidity from the air and will sour in no time. It will foul up any lawn mower that is left sitting with unused fuel in it for more than a month. Alcohol is also hard on some fuel lines, degrading them and causing them to leak in short order.
So, that mower you had with 5 year old fuel in it very possibly had better fuel in it than your newer mower. Bad fuel wrecks any engine, and you can't blame the machine for your lack of maintenance.