Subject: What causes cells not to reproduce perfectly?

I'm sure there is no really simple answer to this, but, as best as 
possible, what is it on a cellular level that causes entropy or it's
like to cause cells to decrease the perfection of their reproduction.
On a simplistic level, it seems to me that if we could cause the cells
to reproduce like they did when we were twenty, barring accidents or
extreme diseases, we could live hugely longer life spans.  Is this a 
built in mechanism???  Do cells know how many times they have reproduced?
Why SHOULD they get worse and worse?

A corillary question is the confirmation that all the cells in the body
with the exception of nerve cells are continually regenerating.  Brain
cells or other nerves do not reproduce - is this correct???

Re: What causes cells not to reproduce perfectly?