TheRealGOAT wrote:
Michael Phelps broke the world record in the 200-meter butterfly when he was 15 years old.
Yes, it has happened frequently in swimming, primarily in the women's distance races where teenagers are dominant. I'll just list the most recent example of Katie Ledecky breaking the world record in the 400, 800 and 1500 while still in high school.
I believe Missy Franklin did the same thing in the 200 backstroke, which may be even more impressive given the shorter distance and therefore greater number of quality competitors who specialize in that event. Franklin's 200 backstroke time from London 2012 still stands and is rarely seriously threatened.
You mentioned Phelps' 200m butterfly record. The current women's world record in that event is considered the most untouchable record in the sport. It was shattered a couple of times by a relatively unknown Chinese swimmer in 2008 and 2009. Some posters on swimming forums think the record will last another 20 years or more.