There is no such rule as the one that would seem to be used in Google Translate: no reason at all to use “6 ans” and “7 ans” versus “5 années” or “8 années.” In this case, the iPhone localization system has the advantage.

Unfortunately there are no simple rules that would simply lay out the use of “an” vs “année.” Along with a few rules, there are fairly long lists of specific cases that require “an” ou “année.” There are more cases where “an” is more appropriate than “année” rather than the opposite. So I think the default is best chosen as “an.”

Here is a summary thread on wordreference discussing the subject: http://forum.wordreference.com/threads/an-ans-ann%C3%A9e-ann%C3%A9es.9547/