It has not always been the case. My sims have previously always started with maximum YA days (and i play with aging on).
Note for challenge team- If the devs are trying to pull the wool over everyones eyes, so to speak and claiming this is "intentional"- For all the challenges played in 2017 and 2018 by the challenge team people always finished on the exact same day before. Now, if this is "intended" some people will have more days than others in challenges, therefore may need to consider adding a rule for 2019 season to allow people to reset to maximum days at start of challenge file.