adobe certified associate

Check out the prep guides (should be here: Adobe Education Exchange​) to give you an idea of what's expected, but as you've alredy been informed, you'll need to be on a very recent version (CC2015+ in all likelihood).

If you're still in education, there is pricing available for you that has a huge discount; otherwise, you can get a seven-day trial—but that's a steep climb to prep for the ACA tests in more than one product.

To fully prepare, you should have knowledge of the current versions. Tests are based on the current version. You will be able to answer many of the questions using your knowledge of CS5, but those questions that cover new features of CC will be unfamiliar to you.  You could purchase a subscription of CC. If you're not ready to do that, some city libraries have the current versions of Adobe CC software.

Your clients would expect you to have up to date knowledge. Knowledge of old software may be a bonus too.