I just purchased a used VP 150 and was wondering how I can find out how old the speaker is

Call Axiom with the serial #.

Rick is right. Also, if you check the label, does it say v1 or v2? That would be the version, with the v2's being release a few years back.

Since the speaker is made of a wood product, you could try cutting it open and counting the rings...

[biology joke]
feeble, i know

I liked it.

Yup, just call Axiom and give them the serial # on the back of the speaker or send them a email with the serial #. All the appropriate information should be in their system such as the date the speaker was originally purchased.