That depends on who old/used the type is.
A good test is to take a penny and place it in the tread grove with the head of Lincoln facing down. If you can see the top of Lincoln's head you need new tires. If the tread is worn more on the inside or outside you need a front end alignment. If the tread is worn on the outer edges you are running the tires with too low air pressure. Worn in the center, too much air pressure. Of course dry rotted and cracked tires need replacing no matter the tread depth.
If you rotate the tires every 10,000 miles, and keep the air pressure at the automobile manufactures recommended level you should get 40,000+ miles out of a decent set of tire. Replace the tires when they are worn to 2/32" tread depth.
Air
they are filled up with regular air that our tires use but some cheaters in monster truck racing fill their tires up with water to make sure they land on all fours.
Filled with air
If you're having to put air in your tires frequently then your tires probably need replacing. Also if the tread on the tires is worn down they should be replaced (a symptom of worn tread is the tires squealing on the road for no apparent reason).
pneumatic
Helium is lighter than air. So, tires filled will helium will help the tire rise up in the air.
They are filled with air, which is a mixture of gasses, so yes they are filled with gas
Aircraft tires can be filled with regular compressed air, however pure nitrogen is also often used.
so you have grip on the road.
Front and Rear tires - 10 to 14 pounds for the soft tread turf tires.