*devilish" YAsooooooooNBU.
I have a brother with down's syndrome and have experienced all the usual sterotypes - "he must be so loving" "oh he will be a joy to have around, they're always so happy" and "he'll love music then?"
For the most part yes, he is lovely, he's one of the nicest most genuine people I've ever met. But he can also be an obnoxious, extremely challenging bugger!
Life with DB was incredibly challenging until he was about 16. Then things seemed to settle down, he moved on to a college teaching him basic life skills (DB's learning and behavioural difficulties are complex and on the mod-severe end of the spectrum) and started exploring his new interests - like cooking, and a local drama group. Finding he enjoys these things has given him hobbies outside the home, and a group of friends with similar interests. He's 21 now and has a better social life than I did at that age.
I remember when DB was about 12, it seemed like there was no light at the end of the tunnel but we're out the other side now and it can get better. Of course DM and DF still have bad days with him full of temper tantrums and massive strops, but they're getting fewer and further between.
Incidentally, DS and Autism aren't mutually exclusive, DB is on the spectrum. It can be difficult to get a diagnosis as there are many features of downs which are also autistic behaviours (hand flapping, body rocking for example) but it's worth looking into.