The oldest machine that can netboot using the bootpicker is 2008 (only local drives shows up on older machines). But you should still be able to use the N key if you set-up the nbi file as the default boot image in Server app's Netinstall service. If you only have one nbi file then it should already be set as the default.
Alternately, if you still have an operating system in the 2007 machine, you can select the netboot image in the System Preferences/Startup Disk. You can also boot to an external volume if you already wiped the internal disk.
