You have multiple things going on that could or could not be related.

What do you mean by "even above my age group"?

Is the basilar invagination caused by injury, illness or was it present at birth?

Did you sustain a head injury?

Are you being treated for a head injury?

Are you being treated for any inflammation?

You possibly need genetic testing or your neuro should be consulting with a geneticist.

Last but not least, with respect, your use of the word "retarded" was used/termed in an
uncompassionate and apathetic way...With consideration for all with a disabilty, please be respectful of the term. If you want to get a little understanding and empathy, you've got to learn to give a little of it yourself...Just sayin'

It perpetuates the sterotype and false stigma of having a brain injury or disorder.