At this point, there is no way to specify a certain age using the CustomZoi cheat. There is a wishlist post by @Samuriko to allow specifying age of a Zoi in CAZ.
Give us an option in CAZ to input an age number for our ZOI
Perhaps I should change the title of the original post to “how to change the appearance and the life stage (and not age)” 
At this point of time, the CustomZoi cheat allows to change only appearance and the life stage; but not the age or number of days within that life stage. It is a front end to enter CAZ. If age could be specified in CAZ as wishlisted, then it could be used with the CustomZoi cheat to position a Zoi at a certain life stage and age.
I think stopAging cheat works only on the active playable Zoi, not the whole family. Though I haven’t used this cheat for months, so I could be mistaken.