Here is my super short super easy gift guide to buying for autistics. There's only one step so pay attention.
1) Ask Them what they want.
Yep that's it. Just like you would for anyone else. Ask what they like. If they cannot tell you ask those close to them. Ask if they have a wish list. If all else fails gift card. While a gift card may not be the most personal it is much better than insulting someone by buying something years below or years above them.
A while back I wrote a post about gifts in our house. You can read it Here.