Apparently there is a neurological connection with incontinence and autism.
That said, I think if adults find comfort in a type of underwear that is hardly going to be seen by anyone else, then the problem is?
In a humorous light, I imagine that nappies would actually be far more comfortable than regular underwear; imagine sitting on a hard bike saddle with that extra padding for comfort.
We have friends with a lad, 15 now, who is Fragile X, has always been in nappies and likely always will be.
Maybe whether for a medical reason or not, if a person wears nappies as a preference, I don't think 'weirdo' at all, more that there are far more important things going on in the world to be worrying about.
Each to their own.