Being the firstborn of eight, I can tell you all that there are exceptions...
I was born near the middle of a line of nine children. Every single one of us is/was pretty smart, but there seems to be no correlation between birth order and level of intelligence. Also, formal schooling performance is varied throughout the strata.
Both of my parents were very bright, both were voracious readers, and my mother talked to us a lot. Oh, and none of us attended school before grade 1.