Hi! I am Justin. Please give me a few minutes to go over your question.
I would suggest transitioning to another software entirely. Apple did away with aperture and so any support from apple is null at this point. With that in mind any time you open up an Aperture Library on a mac it need to be the current version that the library was created on or newer. If it is opened in a newer version of Aperture it will save in the newer version and there for will not work with an older version. 3.6 is the latest per the Apple release notes. https://support.apple.com/en-us/HT203265As far as upgrading I do believe that Aperture will continue to work with every current version of Mac OS.Also 10.10.5 is Yosemite.