Actually kids are not usually diagnosed with bi-polar until they are teens, and then I think it's a bit sketchy.
Anybody that has teenage girls, or has had the great pleasure of spending a lot of time with one would think they are wacked most of the time. My daughter (16) will be jumping around the house one minute acting like a fool and then 2 minutes later she will be moping around the house like she's all depressed. Then 2 minutes later she's bubbly and cheery again they are moody, not mentally ill. It is very situational.
The new diagnosis, at least in my area is bipolar 2, which a lot of psychiatrists are labeling teens with. It's basically the same thing, but someone with bipolar 2 can cycle from the manic stage to the depressed stage much quicker.
Granted some of these kids that are diagnosed early in life will go on to grow up to be an adult with a mental health disorder, but many will come out of it when their hormones straighten around.
I have never heard of a baby not crawling and later being diagnosed with a mental illness, but I suppose it isn't out of the realm of possibility, what next?