Full disclosure, bdunn's firm built my app (@aeir_talk), and take it from me, they make great products. I am so excited to see them putting their own products out. Especially one that is meeting a critical need for its users.
To be honest any age group can use this. You can make the flash card's as hard or as easy as you want. One of our customers purchased the app for her 14 year old child. You can read her son's reaction here: http://hamptonroads.com/2011/12/chesapeake-father-creates-ap...
We are brand new but the feedback I am getting from therapists working in the age group 18 months - 10 years and in the moderate to severe category of Autism. Older children with higher ranges of Autism are still using the app. Rather than words however, they are using it for social interaction (learning visual and emotional cues) through the flash cards.