See
Java for Mac OS X 10.5 Update 1 adds support for Java SE 6. Note the system requirements exclude PPC machines.
Apple does not drop software support, however, they stop updating older versions because the requirements of new software exceed what older hardware can provide. This is simply the result of obsolescence. Your version of OS X is capable of supporting Java SE 6, but your hardware is not able to run that software.
If it's essential that you be able to run new versions of software that have requirements that exceed what you hardware or OS can support, then you have to upgrade accordingly. Sell your PPC machine or donate it to a reputable charity or local school/church. Then buy a new machine that can run the software you wish to use.