Not so much any more, max. No one wants their kid to be the youngest in the class, so parents are delaying starting school.
Actually, it's okay. What used to be taught in kindergarten is now done in preschool. They start kids reading in kindergarten now, which used to be started in 1st grade. Kids are all over the place maturitywise at that age.
Full disclosure, I have a September birthday. I was 5 for a couple weeks in 1st grade (i.e., I turned 6 about 2 weeks in), 16 when I started my senior year of high school, 17 when I started college. I was not unique in that regard. The cutoff in the Connecticut town I grew up in was December 31; I had a friend who didn't turn 6 in first grade until after Christmas.
Weirdest thing happened on the night before my 18th birthday. Somewhere around 9 at night, I got my finger slammed in a big heavy dorm door; I went to the infirmary and they sent me on to the emergency room for x-rays. Except the ER couldn't treat me, because I was technically still a minor. My parents were moving that weekend, so there was no way to contact them (this was waaaaay before cell phones). So the ER docs waited until midnight (so I technically had turned 18) and I could legally give consent.