According to an informal and thoroughly unscientific survey of young American women, the most common age given for the last spanking from a parent was 17. The oldest age given was 22 by a young woman spanked by her father for too much partying.
My wife received her last parental spanking when she was 18. For our daughter, it was 17.
American boys tend to be last spanked by a parent in their late preteens. I was 12.
For anyone whose interested, here's one explanation for the disparity in ages.
Once boys reach puberty, they are more likely than girls to treat getting spanked as an attack. They tend to blame the spanker more than themselves for the spanking.
Girls are just the opposite. They are more likely to blame themselves for having to be spanked. For parents, this not only makes girls easier to spank, it also makes spanking teenage daughters more effective than spanking teenage boys.
Of course, there are always exceptions to these basic observations.