Change meta_value after query

I am trying to sort by meta_value, but when the data was entered into WP
it was done wrong.

Here goes.
I have a custom field meta_key = age
meta_value = 05/06/2000

I am using $query = $query_string .’&meta_key=age&order=ASC’;
to pull the data.

What i need to do is after that query I want to use date(“Y-m-d”,strtotime())
to reformat the date to look like 2000-05-06.
Once that is done I then need to sort the post by that meta_value. ASC

So right now i can pull that data but am stuck on what to do after the $query.

Thanks for the help.