My son is 6 and was diagnosed in May. I told him a few months ago about the diagnosis, although I didn't refer to it as ADHD. We talked about some of the trouble he had in school, and I mentioned that his brain works differently than some other peoples' brains. My husband also has ADHD, so I told him that he inherited the same kind of brain that daddy has. (He adores his dad, so he liked hearing this.) I recommend reading, "Superparenting for ADD," as it had great suggestions for discussing ADD with your child. As the book suggested, I told my son that he has a super-fast Ferrari brain but that his brain has bicycle brakes. His brain is so powerful but he needs help stopping it since the brakes aren't as powerful as they need to be. When we started medication last month, I told him that the meds would help strengthen the brakes so that he could slow down and focus more at school. So far he hasn't seemed phased by the revelation that his brain is different and actually sounds proud when he talks about his "race car brain."
Again, I highly recommend this book:
http://www.amazon.com/gp/product/B001NBEWLI/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=1535523722&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0345497775&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=09K06SRBXNGFCPP8QGTQ
It helped me stop focusing on the negative things about ADHD and to embrace all the great "mirror traits" that kids with ADHD often possess.