D.K.
I would agree your older kids are a little old to be going. I think at this time taking their younger siblings/nieces or nephews and going with them it is fine.
I remember my mom pulling the plug on trick or treating when I was in seventh grade. I was mad but kind of understood. I found it way more fun in the long run to answer the door.
As a parent now too when opening the door and finding these big teenagers at my door kind of puts me off a bit. It is one thing if they are accompanying smaller kids but to see a gang of 13+ year olds at my door I just don't care for it and think they are too old! :)
Since your son is autistic it may be harder for him to grasp. If you are going as a family out and all that I see no reason for him not to dress up too and maybe just have extra candy for him at home.