Measured by quote activity at Hagerty, the type of people showing interest in modern classics (collectible cars with model years from 1980-2009) is basically ordered from youngest to oldest. That Millenials (born after 1985) and Gen Xers (born 1965-84) together make up two-thirds of the people interested in modern classics isn't really surprising. Certain modern classics have the same kind of nostalgic appeal for younger enthusiasts that muscle cars have long had for Bay Boomers (born 1946-64) and 50s cars have for people from the Greatest Generation (born 1930-45). Those age groups are likely sticking to what they've liked for a long time, as they have less of a connection to more modern vehicles.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2015/09/18/graph-of-the-week-what-age-groups-are-buying-modern-classics