Continuing from another discussion:
I found a paper on synesthesia from 2014 from the ideasthesia Wikipedia page. I only read part of if so far, but it looks interesting.
Currently, little is known about how synesthesia develops and which aspects of synesthesia can be acquired through a learning process.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4137691/
Just a side note: in addition to the colored magnets study , number-form synesthesia can be based on clock faces like this (1-12):
That doesn’t mean that there isn’t a narrow window of time where synesthesia has to be developed, but it seems to happen after birth.