This is what I found upon looking up.

The Danish tradition of covering a 25-year-old in cinnamon (and a 30-year-old in pepper) if they’re still unmarried traces back to the 15th–16th century pebersvend — a wandering spice merchant whose itinerant trade kept him from settling down and marrying. The cinnamon-specific custom is much younger: the earliest documented cases come from the 1960s in Northern Jutland, with the folk explanation that friends simply didn’t want to wait five more years before “spicing up” their single peer, and cinnamon was a gentler substitute for pepper. It spread nationally by the 1980s and remains strongest in Jutland and Fyn. In practice, friends surprise the birthday person, tie them to a tree or lamppost, wet them down, and shower them in spice — a playful, consensual ritual rather than a law, though popularly dubbed the “Age 25 Rule.” The 25-year-old earns the title kanelsvend (or kanelmø for women); at 30, pebersvend (or pebermø).