When you change the age settings, the change does apply to all your saved games. This only matters if you forget to change them back when you switch from one of his saved games to yours. Technically, it only really matters if a sim has an age transition because otherwise, as soon as you change the settings to your own, the days left in each lifespan adjust accordingly, but only within each age group, i.e. if a toddler has 5 days left and you make the span longer, the toddler will then have more days left, but only if he's remained a toddler and not yet become a child. I would make a habit for both of you to check the settings as soon as you load up your neighborhood.