In many places, the government forbids adults to view such material or restricts their viewing it. Parents don't let their children gamble. Governments in many locations forbid adults to gamble or restrict their gambling. Parents make provision for their children's future until adulthood. The government makes provision for adults' futures by taxing them under Social Security.
Parents provide medical care for their children.
The government provides it for poor and elderly adults and seems intent on providing it or medical insurance to everyone else. Parents educate their children, sometimes requiring them to do things they would rather not do. The government requires adults to send their children to school, if not a government-run school then an institution that satisfies the government's definition of school. Adults are not responsible. Adults don't know how to conduct themselves.