My 13-year-old nephew didn't talk until he was almost 4. Up until that age he grunted and said "uuuuuhhhh" A LOT! There was a lot of concern among the family, but no ability to find out anything/have any testing done as his older brother had a genetic disease that took all the family resources (extended as well as immediate)...both financial and emotional. This nephew passed away at 6-years-old. Anyway...nephew who didn't talk was about 3 months shy of his 4th birthday when he looked at his mom and said "pretty sky (pointing out window), play outside?" His grammar wasn't perfect, but his speach was very clear! From that day forward he hasn't stopped talking...mister motor mouth to this day. He is now starting 8th grad with an A/B avarage and has signed up for debate as an elective. We do tease him about "uuuuunnn" though!

It is true that every child develops differently. It is also true that the environment plays a big part in a child's development. Do people read to your cousin? Do thy talk to him regularly? Are his needs met without him needing to "ask" for anything? What I mean is, if he toddles into the kitchen is he handed a drink or something to eat? If this is happening, he may not see any need to communicate yet.