I think they say bone ossification happens between 4 and 6 years old.
My Human Oesteology texts say that vertebral ossification happens between THREE to six years old. Actually, there are 2 or 3 centers of ossification for each vertebrae - depends if it's cervical, thoracic or lumbar vertebrae - and the age of ossification for each center and each vertebrae varies. Eg. the neural arches of each vertebrae fuse between 2-3 years, but then they don't fuse to the body of the vertebrae until 3-6 years old. Some cervical vertebrae fuse earlier than others - eg. C3-C7 neural arches fuse to the vertebrae body by 3-4 years old but C1-C2 aren't ossified until 5-6 years old. All this comes from "The Osteology of Infants and Children" by Brenda J. Baker, Tosha L. Dupras, Matthew W. Tocheri
Remember that it is a process, and that more of the bone is present with each year. So, a 2 year old has more ossification than a 1 year old, a 3 year old has more than a 2 year old. You can also see the line where the 2 bones are joining into one bone. And how complete the line is, is how ossified the 2 bones are. Sometimes it's really obvious, and other times there is just a faint line showing where the 2 bones are joined, but not quite complete hence the line. When you can't see ANY line, is where the ossification is complete.
Without an x-ray, you don't know how a particular child is developing. For me, RF to 3 is the minimum, and anything after that is bonus.
Also, the vertebrae are still forming through out childhood, teenage year and into early 20's. After age 22 or so, then the vertebrae look like yours or mine, until then, they are still not completely formed.
So for going a little OT, the body is pretty amazing...