I have multiple save files and they all have different age settings that stay the same as were initially set when I open them again. If I start a brand new save, it will have the age settings of the last save file I played. Once I change it to the age setting I want for the new save, it stays that way. It's always been this way for my game since I started playing in 2014.
If your previous saves (i.e NOT a new save) age settings are changing, it must be a bug or glitch. I would suggest doing a repair on your game and see if that helps. If it doesn't help and the age settings are still changing, perhaps you need to report this on the EA Answers Headquarters site.