My guess would be it is correlation between giftedness (has a large genetic component, gifted adults tend to be more educated, educated parents, especially women, tend to breed later) and "advanced" parental age.

My other guess would be that correcting for education level of parents you would see slightly more gifted kids with *younger* parents, and also less special needs/2E kids.

But my belief in the higher quality of younger eggs/sperm didn't lead me to reproduce earlier (we married at 23/24 after finishing graduate studies but before starting our careers, kids born at 30/32, 33/34 and 35/36). Of course as of now none of my kids have tested as gifted either. As for myself, I was born when my parents were 26/36, am the eldest of five and was the only one id'd as gifted.