Sometimes the MC trigger doesn't happen right away and inactive sims have to be off-screen ("home" or the equivalent) before it does.
But given that the sim was already age-stuck, which means one or more sim days beyond their expected birthday, possibly there is another cause. One common one is that there is something wrong with one of their current outfits or outfit assignments (try changing them all). Or they already have more traits than they are supposed to have in the current age stage (use MC Cheats to remove any excess ones). If the sim is a Role Sim, it could also be that they are being pushed to staff their assigned station (cash register, bar, etc.) at the same time as they are being pushed to age up, with the net result often being that they can do neither -- sometimes it's just easier to TA these sims if we don't even know who they are or shove their ages back down using MC so that they aren't age-stuck anymore.
Any of these things possible?