Most babies start to roll over between 4-6 months of age. They straighten their legs between 4-8 months old. A lot depends of "presentation" meaning how they were born--face first vs. feet first. The legs are folded the last few weeks of birth therefore, some children seem to show this more than others. Ask your pediatrician about it.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics