What they don't know can't hurt them, I would say if you are comfortable and are able to hide this from them, then yes by all means do so- it is your right to be silent.
However, on the other hand you are 21 years old, they have no right to tell you how to live your life and I feel like you should not really care what your parents think, it is your life;live it as you please.
Do what is best for you at the end of the day, it all depends on whether you are living with them or not, I assume you are otherwise this wouldn't even matter. If you start buying magickal tools etc. the conversation is much more awkward if they are found after being hidden rather than if you upfront explain things in a calm, rational manner.
But I don't know your parents, you will know when the time is right, if ever, to tell them.