I have an oldish MacBook Pro and want to buy Scrivener but the latest version is 3. It says it's suitable for OS 10.2 or later. My MacBook (which won't now update any further as it's late 2008 model) runs on version 10.11.6. Will I be able to buy it and use it on this machine. My Mac itself is fine but I do most of my writing on the MacBook. Any comments would be very useful. Thank you.
Not to sound rude, but you mostly answered it in your own question. You have a lesser version than the requirement. Sometimes this means the application will not run at all, other times it means it will run but with errors and often time not worth the hassle of the errors. I would not recommend it. I have personally found Pages to be adequate for my writing needs.
If you really, really want a software, you can also check out novlr (I have no affiliation with them). It's a simple web based SAAS that allows you to have version control, back ups to dropbox/google docs, word counts, chapters, distraction free writing, night mode, notes. The only issue is that they do not provide any form of pre-writing tools such as story boarding, character profiles/relational trees and the likes. Since I am a discover writer, I don't need a lot of the pre-writing fluff and found this tool to be adequate enough.
Back to your main issue though, I would not recommend it. Software is also something hard to get a refund for and you could be out 50 bucks. If you are willing to take that kind of risk then by all means give it a try. My macbook pro is relatively new (a 2014 model) so I can't really provide any personal experience.
Likely worth verifying with the Scrivener publishers and/or Apple, but in most software version numbering systems, 10.11.* is newer than 10.2.*. That would mean Mac Scrivener will work as well on your Macbook as it would on your desktop Mac (presuming adequate processor, RAM, and storage available).
To follow what other people have said, it should work considering 10.11.2 is newer than 10.2. 13 years newer, actually. See the releases section here. I use Scrivener on Windows and highly recommend it.