While I strongly disagree with the general attitude on this site towards adult fiction, I do understand the complaint that most fantasy adventure series are about teenagers, and therefore really recommend The Chronicles of the Imaginarium Geographica by James A. Owen. They are written as adult fantasy, but generally marketed towards a middle-grade audience, due to their style and vibe.
The books span decades (centuries, actually, if we count the time travel and alternate dimensions, etc.) and the three protagonists are full blown adults. In the first book, they are, respectively, 19, 25, and 31. Three young men, who meet under mysterious circumstances, become great friends, and dive into many magical adventures. These adventures technically happen over the span over many decades – they have jobs in the meantime, wives and children, etc. Decidedly NOT in a “baah, boring adult life NOW it’s magic time” but they are actually just adult men with adult lives and families and jobs they love, who also have magical adventures.
And there are plenty of other characters, of various age groups, like one who is a young girl in one book, and later a young woman in later books, and many other adult characters who happily go along in fantasy worlds.
The stories can get really dark, are sometimes very complicated, and contain many allusions to literature, history and mythology. But there’s also battle goats, flying ships, pirates, talking animals driving fancy cars, lots of humour, and generally things one would rather find in children’s books.
So if you want to read about adults having fun and whimsical adventures, look at these.