Is it possible to develop synesthesia without the "aid" of brain trauma before the age of 21?

Reasons why you would try to:
>Better memory
>Unique perspective
>Physics or math could become more intuitive (source: Richard Feynman)
>X could become more intuitive

A possible way of developing it is to start attaching X sensory information to Y sensory information. e.g. Attaching some color, sound, emotion, bodily feeling to a letter. This is just hypothetical and I don't know if it would work. Feel free to contribute some of your brilliant ideas.