The maturity of a "grown up" is nothing fuzzy, foggy but is clearly expressed and can therefore be observed, measured, taught, learned and valued!

A mature adult human being…

… recognizes the correct stage of his/her maturity as an adult.
… aims to get more mature as an adult human being at any time.
… knows the difference between urgend and important.
… knows the difference between doing the right thing and
    doing a thing right.
… learned to admit mistakes and to apologize for them if appropriate.
… learned to listen actively to others.
… seeks to understand before trying to be understood.
… is trueful to himself/herself and to others
    (stops lying to himself/herself as well as to others.)
… takes the full responsibility for his/her life.
… can express his/her needs to fellow humans.
… aims for its own and fellow humans mental health
    (e.g. stops playing psycho-games).
… aims for its own and fellow humans bodily health.
… can forgive himself/herself as well as others.
… frees himself/herself from any kind of
    material or mind prison as far as possible.
… knows to put himself/herself in others peoples skin or situation.
… learned that meaning depends on context and knows therefore
    that there exist many truths (contexts are endless).
… knows that healing/building is from a memetic point of view far
    more complex than hurting/destroying and therefore much
    higher in hierarchy.
… knows the difference between magic, mythic and mystic.
… understands that life happens here and now.
… meets every human being on the same level like him-/herself
    without lowering nor aggrandizing him-/herself nor the other.
… learned to control his/her central nervous system (non-vegetative
    nervous system, vegetative nervous system, brain etc.).
… learned to control the RQ world with the LQ world and vice versa.
… accepts its own death (complete end of body and ego) entirely as
    lifes conclusion (without gloryfing it!) while saying yes to life
    at the same time (Zen: Buji).
… can answer all questions of the master correctly without hesitation.