As far as I know, the DNA sequence becomes shorter every time a cell divides.  

A shorter sequence results in information loss and aging characteristics.  

As a countermeasure, shouldn't one stay young and healthy at least longer, if the DNA in a cell is changed with a "better" DNA (from years ago... let's say your DNA was extracted with the age of 20 and you reinsert it every 10 years or so)?

If this is true, then (as for today's technology)  

- is it already possible to perfectly reproduce the DNA? (I only found articles about XNA)  

- is it possible to easily replace the DNA in all human cells? (I found articles about bacteria inserting pieces of DNA into human cells)