when do babies start rolling over? From back to side/stomach... or stomach to back?

4-5 mts

Depends on the kid. My daughter started that at 7 months and my son started it at 4 months.

both mine did at 3 months. my first had them both down by 4 months but my 2nd is still working on back to tummy, he is 4.5 months

My 2 month old can go from his back to his side

Yeah 4-5 months is average. Mine did it once at 3 months and then didn't do it again til around the end of her 4th month.

I've recently read in a baby book that some babies skip rolling over now that almost everyone puts their babies on their backs. It's harder for them to roll over from their back to the tummy so some don't roll over until after they are already crawling. My daughter is 3 months and she doesn't even attempt to roll from her back. When we're doing tummy time she does start to roll but she doesn't even get halfway there, yet. My SIL's baby started rolling at 4.5 months.

my daugter was about 4-5 months when she started rolling over and about 6 months when she mastered it. i wouldnt worry about it.