Why does the Earth still have a magnetic field if the Earth is so old?

You've been misinformed.

Hi. Because the core is made of iron and is rotating. Also a magnetic field does not fade over time. It can change to electricity and back, but conservation laws hold true.

The Earth's core is made of SPINNING iron, and the field is in the process of reversal, hang on to your belt buckle!

I guess your understanding is wrong!

The earth's core is made of Iron. This generates a magnetic field.

Actually its mostly constant bro. Nuclear fusion is taking place in every star in the sky. In the center of the earth, nuclear fusion took place but with less hydrogen so we did not become a star. Instead we become a hard mass with a hard nuclear reactor in the center, it does not grow nor shrink it just is. This is because the specific heat is so low that it can not grow anymore only burn at a constant rate. IF you remove the top layers of the earth all you are left with is a star like matter. Its like a round cake full of icing, remove the crust as a whole and your left with the source of the power. Now the only way that source can become crust again is if it is cooled to a certain degree, this is an extraordinary happening, which has already taken place, The main factor of all planets is the source in the middle, the more nuclear fusion taking place the more distort and warm the crust will become. And the less it is the colder and firmer the crust will become. That's why we have ice planets and lava planets. I think some where out there the chances of just a perfect crust is relative, such as ours. So really the magnetic field stays the same most the time but can decrees and increase it all demands whats going on in the middle.

The magnetic field fluctuates greatly in the direction it "points" and its intensity. I could go on about this for days, I have a lot to say about the subject, but I'll keep it short. Currently the field is weakening. It might even get so weak that it's negligible. Geologic history shows that it will return, though. There are ways of knowing what it's done in the past, if you know your geology, and it's been really weak in the past. Often, over extended periods of time, the North and South magnetic poles flip over and switch places, or they end up in really odd orientations. The magnetic field is created by the solid iron core and the liquid outer core. It's difficult for us to know up here on the surface what's coming next or why it changes intensity and position. It's much easier for us to detect what it did do in the past. With it as weak as it is right now, we can even track daily changes in the magnetic field. A couple of years ago I saw a really odd display of "Northern Lights" centered at the equator, of all places, that might have been due to the magnetic field being messed up that day.

The earth has a magnetic field because the molten outer core and solid iron inner core is spinning. The weakening of the magnetic field is believed to be happening because the north and south poles are ready to flip. (reversal of the polarity and not the earth actually rolling over) Geologists have found evidence in the past that this has happened several times and it is due to happen again. However no one knows if it happens suddenly or gradually.

I believe that Halflife may apply to magnetism. First off, how do you know that it is as magnetic as it used to be I have heard that it is slowing. The fact that it is eround may help a bit. One last thing, this is just another sapport for the young Earth creationist idea.