Just as a personal data point... I had a plastic tricycle at age 1, according to photos. I don't remember it, it was lost in a house fire when I was about 4. After that I had a Big Wheel and then a BMX. My bike hoarder grandad who built the BMX for me refused to put training wheels on it. He put shorter cranks on it and had me climb on from a curb. I remember it being really awkward. But I must have made it by about 6 because I rode it to first grade, and I do remember graduating to longer cranks.