Obviously, you know nothing about how aircraft are maintained.
Wether the fire was caused by tires or brakes is irrelevant to the plane's age.
Trust me, neither the tires nor the brakes on that plane were 20 years old. Other than the hull itself, probably nothing on the plane is 20 years old.

You see, airlines maintain their planes much differently than you maintain your car.
For example, you probably don't replace your alternator until it fails. You might keep your car 6 years and had to have replaced a failed alternator twice. Or you might sell your 6 year old car with its original alternator. You just never know. Airlines don't operate on the "you never know" philosophy. They will replace their alternator once it has had "x" hours of service. Even if said alternator is in perfect working order.

Yes, just like any piece of machinery (and your own body), an older plane requires more upkeep than a new one. Many factors come into play, but it's usually cheaper to spend money on keeping the old plane in good condition than it is to buy a new one.

That concept has changed the past few years. The thinking now is that it's cheaper to buy a new plane than to keep the old one.
But it isn't the maintenance cost that changed the equation. It is the rising price of fuel.
The older planes simply burn more than the newer models. It's all a matter of perspective. AA and DL bought MD-80 series aircraft because they were far more efficient than the 727 (which AA had bought because it was way more efficient than the 707 ... ditto for DL and the DC-8).

It will be up to the NTSB to decide (not you or me) but I'll bet the age of this airplane had nothing to do with the fire.