So it appears there are a lot of questions requesting the ability to boot from usb on a Macbook. Example and here
Now I believe these questions are missing 1 important point and that is that I believe it isn't possible on older machines. So what are the first year macbooks that supported booting from usb?
Best Answer
Short Answer
Any Intel Mac capable of running Mac OS X Tiger 10.4.5 is capable of booting from USB (so long as the version of Mac OS X on that USB is at least Mac OS X Tiger 10.4.5.
Since all MacBook models are Intel Macs, the model year is irrelevant aince all Intel Macs are capable of booting from USB so long as they're running Mac OS X 10.4.5 or later. For the record though, Apple started producing MacBooks in 2006 and the original MacBook Pro (January 2006) came pre-installed with Mac OS X Tiger 10.4.5.
You can read more about how to set up and use an external Mac startup disk.
Long Answer
In practice I've found that it's easier to boot from USB with MacBook models from late 2008 onwards, espececially if you want to boot from a USB flash drive. I cannot advise specifically why this has been the case in my personal experience (maybe I wasn't doing it right in those early years of Intel Macs), nor whether other people have had different levels of success. However, since your model MacBook Pro is a 2009 model (based on your comment) this will not be an issue for you. Just be sure to follow the how to set up and use an external Mac startup disk if you intend to do this.
Extra info
It may be of interest to some people that it was technically possible to boot from USB in the pre-Mac OS X era with the introduction of dual channel USB in 1999. Slot loading iMacs and a model of the AGP PowerMac G4 were able to boot from USB drives.
As an aside, you can't start a Mac in Target Disk Mode using a USB cable. To do this your Mac will need to have a FireWire port or a Thunderbolt 2 or 3 port or a USB-C port. See Share files between two computers with target disk mode for more info.