I don't think there is any clear-cut definition, as one person will mature/adapt/change into adulting ways of doing and thinking at a different rate (which can also be highly variable from one day to the next) to another person.
In my case, I don't think I have ever and probably will never be able to fully understand how to do adult romantic relationships (and things associated such as approaching people, dating, sex, etc.) 'properly' though in other areas of life I'm quite adept at 'adulting'. Things like my job (been a railway shiftworker since the late 1980's), and being a parent to my adult kids and 8 y/o daughter, my ability to understand and manage money/finances (though sometimes I doubt myself), my hobbies/interests (of which sex, dating, relationships, etc. is not a feature), my ability to make informed judgements about personal issues like health and wellbeing, etc.
I had no interest in sex/dating/relationships in my teens, or my early 20's. Does that mean I wasn't 'mature' enough until then? Possibly? I'm in my 50's now and after disastrous relationship and dating experiences I'm completely frightened of everything related to sex and relationships, so does that mean I am not 'mature' enough? Not sure.
Does a person ever 'fully mature' and compartmentalise the 'inner child'? I don't think so.
