Ivy was almost four months old when she first rolled over front to back, and four and a half before she went back to front. Ruby as of eighteen days has already rolled both ways. Fortunately we did not discover this the hard way. Don't be too confident about what your kid can't do! I thought it was impossible before a couple months, but our pediatrician saw it coming at the fifteen day appointment and said once in a while kids come out "ready to roll" - actually she didn't put it that way but I wish she had.
This kid is strong. When she's freaking out I'm nervous holding her because it seems like she could make me drop her if she chose to. Look, I'm already afraid of her and she's not even fifty feet tall yet.