Something unknowable in immunology? Simple logic?

[Dina Fonseca]

A recent post stated:

I can see that the rate of antigen experience by T cells is 
greatest when younger, but this ignores the fact that new TcR and Ig
are generated right up until the day you die.  Since antigen receptor
 generation is essentially a random process it is inevitable
that cells with self-reactive receptors will emerge from the bone marrow
 all throughout life. If the thymus involutes after
puberty, where are all these new cells getting their education
 (both in +ve and -ve selection)?


Is this really a problem? If the thymus involutes, then 
no T cells will be made i.e. no positive selection.
As no T cells are made, it does'nt matter that there
is no negative selection either.