This has always puzzled me about the millennial generation. On the one hand, let people enjoy things, and who am I to judge what people like to do? On the other hand, it really strikes me as a form of arrested development and adults playing with LEGO really gives me the ick. That might be projecting my own fear of being Peter Pan, unable to grow up...
Building a LEGO set is akin to building a model car. Adult collectors and builders of model cars, planes, trains, etc. were a thing before millennials.
"When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up" (C.S. Lewis)