sitting question

Stephanie • My names Stephanie, I`m 25, a wife and mom to two kids, and expecting #3!
When do babies typically begin sitting on their own? My friends baby just turned six months and can sit up by himself yet my baby is five and a half months and is just learning to sit up in a walker or against the couch. He is beginning to try and sit up, doing little ab-like crunches but not close to sitting on his own. Is he behind? Should I be concerned?