Bug 16285 - write_age() function throws error for patron categories that don't includ...

Bug 16285 - write_age() function throws error for patron categories that don't include dateofbirth in form

To test:
1 Add a patron in category Library or School
2 Note the js error in the console
3 Apply patch
4 Repeat and note the error is gone

Signed-off-by: Chris Cormack <chrisc@catalyst.net.z>
Made it an explicit if, and fixed the author of the commit

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>