what I need to know is why, why would twin 1 age much more than twin 2 simply because of a change of speed?

I'll probably be advised to read a few more books on special relativity after I answer this, but here goes. Note that this is a highly simplistic explanation.

Special relativity begins with just two postulates:

1) The speed of light is constant in all reference frames i.e. whether you are in motion or not.

2) The laws of physics remain the same in all reference frames.

I now introduce a definition of time as follows: the amount of time taken for light to reach you. This is reasonable: light contains information, with the help of which only can we determine a change in time. Obviously, if light comes to you more slowly, time will be seen to move more slowly, because the information required to differentiate between the past and the future is taking longer to get to you.

Now suppose you are at rest. Light hits you constantly, meaning that to an observer next to you, also at rest, time will pass at the same rate you measure it to be. His clock wil tick at the same rate as yours.

Suppose, however, that you are now moving with a certain velocity

*v*. Light, since it always moves at the same speed, will take longer to reach you, as it now also has to cover the distance you've left behind as you're moving. Naturally, it will take light a longer time to reach you, making it seem as if time is moving more slowly. Since your clock is also moving at the same speed, it too is subject to this, and will tick slower than the observer on the ground, because it measures time itself to be moving slower.

And that is how time dilation works. Again, this is a highly simplistic explanation, which should enable you to understand it quite clearly.