Thank u for your response.  I found it on wikipedia on knee, while researching for understanding of my own knee probs..  It is under human anatomy.    It states quote, "upon birth, a baby will not have a conventional knee cap but a growth formed of cartilage . In human females  this turns to a normal bone knee cap by age 3, in males by age 5. "  Unquote    Having a very enquiring mind I now want to know why.   Also I came across some other site that mentions some other area, maybe shoulder , that says some ossification happens in human females at age18 but in males at 21. Try to find it asap.    Thank you Eileen

Hi Eileen,

There are general skeletal developmental differences between males and females, not just isolated to the knee cap.  It appears that these may have developed due to evolutionary selection.  The differences between males and females can be used for sex identification in forensics.  You may find the following links interesting to read:

Agreed David but Eileen was specifically asking about differences in ossification of the patella in pre-pubertal children, whereas the links above are in adults where sex-differences are, as you say, very common and used forensically. Having looked I am unable to find a good source for the statement in wiki and am unsure whether it is correct. I will discuss with one of my consultant colleagues who is a paediatric radiologist to get a definitive answer whether it is correct or not and will then post again.

The information I have back is as follows:-

It is true that ossification of the patella "starts" between the age of 3 and 6 years but it is not "complete" by those dates.  Ossification is extremely variable, however females are usually skeletally more mature than males from an early age, the reasons are as yet unknown.