The aging checkbox in Game Options is all or nothing. When it is checked, all sims in town including the ones you are actively controlling will age. When it is off, none of them will age. Pregnancies will still progress to birth, but the babies will remain at age 0 if aging is off, adopted children will remain the same age, etc. You can still age a controlled sim up manually to the next age stage by presenting them with a birthday cake even if aging is off, though.
One slight mistake many players make is that they will re-engage aging when a sim is already on the last day of their current age stage and then present them with a cake. If both are done in rapid succession, then the sim may age up twice in the same day thus skipping a stage entirely. And that's especially not pretty if they were an Adult to begin with as the second age-up will be to the next world (they will die of old age) rather than Elder.
In order to have selective aging through which we decide which sims are aging and which are not, mods must be used.