They changed the law in about the late 70s I think because my son who was born in 1979 got one when he was born (or got the form in the hospital) as I recall.
At some point the tax laws changed. The IRS started requiring a SS# for every child you deduct on your tax form. That is why kids get a SS# right away, so you can get your tax deduction/credit. I do not know when that started.