kagis
According to industry reports, guitar sales have decreased consistently for over a decade. Total US guitar sales dropped over 50% from 1.5 million units yearly in the 2000s down to around 600,000 as of 2020.
However, the past few years showed a leveling off of declines, and 2021 even saw a slight uptick likely driven by pandemic factors. But regardless, the market has gotten undeniably smaller since its peak.
That doesn't have a breakdown as to age of buyers, but if sales are less than half what they were two decades ago, I imagine that playing the guitar is probably less popular than it was at that point in time.