Later start in school, potential extra year 10 before high school, something called "højskole" which is potentially another year or so after high school (look up a guy called NFS Grundvig to explain this), conscription/volunteer for the military for some people.
Most long degrees are 5 years from high school to masters, 3 + 2 officially, but you generally don't want just the 3.
Also it's a reasonably common thing for people to get delayed. Failed exams, stress, even babies. Government is looking a lot at how to get people through quickly.
Age 2-6: Daycare/kindergarden
6-16/17: Grade School (folkeskolen)
16/17-19/20: High School (gymnasiet)
19/20-20/23: One or two years off where people work/travel/move out from their parents etc. (as you can see, we're close to the 22 year average here)
20/23-25/28: 5 years of university - add one year for the standard extra interships and you have 6 years of university
Fastest possible way: Start school at 5, skip 10th grade, just straight to High School aged 15, finish high school aged 18, straight to university and study for 5 years (fastest possible degree in the regular system), and you're 23 by the time you get out of the system.
It's true that 10 or 20 years ago it was normal to spend 6 years on a 4 year study. Times have changed. And if they haven't yet in Denmark, they will in time.