In the US the legal age of adulthood is 18. This is very far from the actual maturity point. It's not infantalizing to be accepting and unsuprised when a 18-22 year old does something childish.
Edit: Being a child, and acting childish is an important part of the life cycle that allows development. We seem to be bumping into some biological limits on how late we can extend it, but if it were possible I would.
When I worked in baseball concessions as a teenager, the employees who were the most responsible and dependable were the university students. The older workers tended to be more experienced, but some of them were a little sketchy and a couple were fired for showing up drunk.
There is a tremendous variation between individuals. Part of the point of the university degree as a credential is to identify those people who are responsible enough to complete the program. I graduated at the age of 21 and immediately became an Engineer in Training, working on code that could kill people if it behaved improperly. I took that responsibility very seriously, and I'm thankful that my employer and coworkers took me seriously in return.
I fear that a focus on age tends to lead to prejudice. If you just want to ensure young adults can be allowed to make mistakes without ruining their lives, I would encourage you to expand your goal. There are many adults who could benefit from a bit more compassion and forgiveness. We don't need to restrict it to the young.
Edit: Being a child, and acting childish is an important part of the life cycle that allows development. We seem to be bumping into some biological limits on how late we can extend it, but if it were possible I would.