People have been asking me if Final Cut Pro 7 works on Lion (Mac OS X 10.7) and/or Mountain Lion (OS X 10.8) and the answer is YES.
Ignore what the employees at the Apple Store are saying. Either they don’t know, or they’re trying to sell you newer computers and software.
The more important thing to watch out for is that all your third-party devices, such as capture cards, will work on the newer OS’s. That you’ll have to research for yourself. But I do have one client with AJA Kona LH cards that don’t work with Adobe CS6 on the newer OS’s. So do your research.