Sims rarely come out of CAS at the beginning of the life stage. Young Adult's start at a random age out of CAS, sometimes at 21 days, sometimes at as few as 12 days to go. I have it happen all the time. Nothing to worry about. Your sim probably just came out of CAS 16 days to go. After all, you said you checked on Day 2, not on Day 1. So techincally speaking, your Sim didn't age randomly, that's just the age they were out of CAS. Make sense?