Kidult

A kidult isn't a kid who acts like an adult; it's an adult who acts like a kid.

Source: tatoeba (6719376)

Organizational methods appear significantly more frequently in kidult programs, next most frequently in adult programs, and least frequently in children's programs. Escape methods appear most frequently in adult programs with kidult and children's programs about equal.

Source: wiktionary

By casting popular culture as simplistic and ‘kidult’, as distinct from the authentic adult culture she approves of, she reduces a complex culture to a generational divide: the new is childish; while the old and established is timeless, worthy of consumption by adults.

Source: wiktionary

In the 21st century, whose “kidult” blurring of the lines sees grown-ups listening to “kindie rock,” feeding their kids gourmet gelato instead of Good Humor, and dissecting the last Pixar film, the notion of a tree house in our collective backyard with a misspelled sign saying “Kidz only” is a healthy thing[.]

Source: wiktionary