Child Development Gap Analysis

My three questions revolve around the idea of having been born to a 40 year old mother. The focus of my questions is on the differences cognitively and my performance compared to my older siblings. Does having a child after 39 affect the child cognitively? Does the genetics lose strength and ultimately decline? The reason for my interest is the giant age gap and the gape in intellectual abilities between family members. My mother gave birth to oldest at age 23 and the middle child at the age of 26. However, I was not conceived till she turned 39 and gave birth at 40 years of age. The gap raises questions such as why do they have a higher IQ than I do? Is there a correlation between the age of my mother and our birth year? Could the gap affect our academic performance and put us behind in
When comparing our educational histories, the first born had the best academic performance, second born right behind the first in performance and then finally myself lagging behind. I was the only person in my family that had to go to speech therapy as a child. Could this be related to my mother having me at the age of 40? Out of 3 children I was the only one without perfect speech and have advanced vocabulary. Is being an older mother creating a setback for the child in development? Is there a correlation between age of birth and cognition? This topic has always been an interest because I always have questioned why I differ from my sisters, particularly in academics. I know there is research stating that having a child as an older mother increases the risk for chromosomal defects. Also, the older the mother the higher risk of having a child with defects in body parts as well. Research gives reasonable assumption that older women are prone to have children with birth