i think its fine to use older material as a source, but not as the only source.
i have used some older material in the past, but it is also important to notice the topics not included and gain those elsewhere.. if a test gains just 5% more material a year, that 5% could be the difference between passing and failing.
The changes in the material from 2015 to 2016 may not be that great, but the older the material the more the changes increase. just my thoughts.