Well now you might have an official reason as to why you're not acting like a mature adult.
People don't become fully "adult" until they're in their 30s, according to brain scientists.
Currently the UK law says you become a mature adult when you reach the age of 18.
Scientists who study the brain and nervous system say the age at which you become an adult is different for everyone.
Research suggests people aged 18 are still going through changes in the brain which can affect behaviour and make them more likely to develop mental health disorders.