I didn't think about it until after my Ceremony of Twelve.
I didn't think about it before my Ceremony of Twelve.
I have seen many times two prepositions of time together. Do they express a point in time more precisely? How the time point expressed in the first sentence differ from the second?
I didn't think about it before my Ceremony of Twelve.
This is a very simple statement; whatever it is, you did not think of it before the Ceremony of Twelve. It says absolutely nothing about whether or not you thought of it after the ceremony; you might have and you might not have, but no implication is made either way (without further context, at any rate).
I didn't think about it until after my Ceremony of Twelve.
This means that you did think about it at some point, but not until some unspecified time after the ceremony. You did not think about it until (some point in time) after the ceremony. But you did think of it.
So the sentence using until after is much more likely. The version with before is a perfectly valid statement, but it doesn't seem very useful. "I didn't think of it before the ceremony." "Okay, but when did you think of it? Or did you at all?" It just doesn't give much information.
The first sentence means that you could not have been thinking of it during the Ceremony of Twelve. The second sentence allows for that possibility, and in fact (at least in my opinion), suggests that you did think of it during the ceremony.
Also, and this is more speculative than the bit above, I think that the first sentence has more of an implication of a missed opportunity. If you say until after, that sounds like you thought of something, but it was too late. When you just say before, to me that is much more of an implication-free statement.
