If I update to Mavericks, will I be able to boot from USB? running a late 2009 (5,5) Macbook Pro. I am currently unable to boot from liveUSB disks, but would really like to. I know that newer macs can do this (Macbook Air, new MBPs), but is it a hardware or a software thing? If I update to Mavericks, would I be able to do this on my machine?
It has nothing to do with the operating system. The update will not change your ability or inability to boot in any particular way.

The stage of the boot process you're talking about is handled by the firmware, not by the operating system.