It was absolutely adult only this summer. In fact, DD said they kept looking at her funny and asking for her room number because they didn't think she was 18 yet. (She was 21, so it didn't matter to her.) Yes, this is a clear change in policy.
ETA - the above was on the Magic in late August, 2018.
I'm thrilled with this! Last summer my college bound daughter (still 17 but so close to 18) couldn't use the fitness center. It was a pain because she plays a sport in college and needed to follow the coaches training plans. This summer on our cruise we might all be able to enjoy the fitness center as a family.