I think you may have accidentally clicked on the timeseries for 55 to 64 year olds, which is understandable because the colors for the two are really close.
55 to 64 year olds have increased dramatically over the past 20 years, due to baby boomers getting older. Like the parent comment said, though, the numbers of 15-24 year olds have been more or less flat over the last 40 years.