It's a little on the early side, but I don't see how it could hurt him any. All babies develop differently. I think 'average' for sitting up is right around 6mos. of age. My older ds was sitting up at 5mos. I think my new baby will sit up early, as he loves to sit up! He has been rolling over for over one month now and his ped. commented that he may crawl early, etc. It seems that people will find something to comment on no matter what.