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.
Modern classics are typically more affordable as well, meaning that they are ideal for Millenials who don't have a ton of disposable income and are therefore faced with limited options for a collector car. There are exceptions, and it is intersting that the proportion of older enthusiasts interested in modern classics is as high as it is, but in general people remain interested in the kinds of cars that are closer in age to themselves.