There's a well-known cultural trope that far more people claim to have attended Woodstock in 1969 than the actual 400,000 who were there. As a symbol of the 1960s counterculture, Woodstock's legendary status has led to a kind of "memory inflation," where many exaggerate or fabricate their connection to the event. It's often joked that if everyone who said they went had actually gone, the festival would have stretched across the entire state of New York.
If we take this a step farther...
One can assume most of the people who attended woodstock were under the age of at the time and that most of the people alive today would have been people under the age of 30.
The US population is about 25% under 30. We could also eliminate anyone under the age of maybe 15 or so since they would be too young to attend, but thats another matter.
As a rough estimate we can assume about 1% of people alive today probably went to woodstock if they were alive at the time.