It's a little more complicated than that, and varies by jurisdiction, but generally, it comes down to whether or not you understand your actions.

Someone with a learning disability - or Down Syndrome, or mental health issues of almost any kind - can often sue to void contracts that they've signed; or their guardian can sue on their behalf.

Courts can rule you to be mentally incapacitated, and you lose your right to vote in many states in the US: http://www.theatlantic.com/health/archive/2012/10/keeping-th...

I'm sure you can continue your research by yourself at this point...