The National Autistic Society (UK) has some great advice for deciding when to explain to your child they are autistic and how to do it:

http://www.autism.org.uk/about-autism/all-about-diagnosis/diagnosis-the-process-for-children/diagnosis-telling-a-child-about-their-diagnosis

They also recommend some books to help you.  I like:


I Have Autism... What's That?

and this next book is useful for explaining to a sibling that their brother or sister has autism:


My Brother is Different: A Book for Young Children Who Have a Brother or Sister with Autism

Carol Gray at the Gray Centre has also produced a workbook for children with autism, so that they can learn about their individuality.  This workbook incorporates the use of social stories.