Some of it's due to size, as much as it is due to the age people are when they have kids. I'm the fifth of five kids with 7 years between me and my nearest sibling. (btw, Dad was 40 when I was born; Amy, the eldest girl...heh, grandma amy, excuse me...is 11 years older than I am.)
She had Rebecca right out of high school (a touch of a family scandal I suppose, though, since this was the late 70s). My dad is the second of like 7 kids. My mom is the sixth of 6, and her mom had like 10 brothers and sisters. We had a family reunion in 2003 for just my dad's brother, sisters, and the various kids and grands... 70 people showed up, 20 of them just being my immediate family, spouses and kids. Our 1980 family reunion on my mom's side (the children of Henry W. Smith) numbered about 160-180. The sheer numbers mean a much higher chance of having both very young and very old parents. Heck, several of my cousins have kids that are close to my age.
large, as much as young
She had Rebecca right out of high school (a touch of a family scandal I suppose, though, since this was the late 70s). My dad is the second of like 7 kids. My mom is the sixth of 6, and her mom had like 10 brothers and sisters. We had a family reunion in 2003 for just my dad's brother, sisters, and the various kids and grands... 70 people showed up, 20 of them just being my immediate family, spouses and kids. Our 1980 family reunion on my mom's side (the children of Henry W. Smith) numbered about 160-180. The sheer numbers mean a much higher chance of having both very young and very old parents. Heck, several of my cousins have kids that are close to my age.