I grew up in quite a strict family, and spankings were a regular part of discipline. Use of the parental hand began around at around two and a half, and from around seven I got the belt from Dad for more serious things. However, in that respect, my little sister got an early start.
This incident happened at bath time. I would be eight and my sister was four years old. Because of our ages, Mom still bathed us together, although I was getting more uncomfortable by this point about being naked in front of the opposite sex. Nevertheless, Mom insisted – I guess it saved time and hot water, from her perspective.