Bug 27530: Set correct dateofbirth for 'C'

Bug 27530: Set correct dateofbirth for 'C'

Categories B, J, K and YA have a upper age limit of 17 years.
We are setting the dateofbirth to today - 15 years to prevent juvenils
to be 70!

Note that DATE_SUB is a MySQLism, but there is no way to be portable
here.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit b647aff01d504533a5a3720c66e6adcf0067792f)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>