As soon as they can read words you can see if word finding would interest them. People who have an activity history involving reading and writing may enjoy it, but some may be insulted by the simplicity of what they can do now.
Family and friends may be appalled by their recognition of how much the cognitive disability has taken away from the person and you need to be prepared to help them with their grief.
Simple words, such as the ones on the cover of this book can be found. Three to five letters may be the definition of simple. I'm not sure if the letter "s" counts or not.

Basic Skills
The category exercises work by finding the first letter and group that fills in each box. When this type of word search is novel to the person, there seems to be less resistance to the words being so simple. The homemade appearance actually appealed to some people by making them feel catered to.
