I do not see the association or assumption that having a responsibility necessarily meant more maturity or deem someone as an "adult".
Kids are a responsibility.
A career is a responsibility.
A relationship is a responsibility.
But would that make a person worth being with?
Would that make a person better than those who do not have those responsibilities that's been measured as societal milestones?
Should one's capacity to responsibility itself tie to one's self worth?
Think taking those risks makes someone feel like a grown up?
Nope.
That just makes them having a responsibility.
Responsibility doesn't make someone a grown up. It just makes them more reliable.
The title itself, one that makes someone question less, whatever that might be (parent, manager, whatever delegate) is an illusion itself.
I say...
To feel grown up is more like a term of stopping associating one's self and rely over a narrative as someone with "having or not having a potential" or some form of youth fragility or sensitivity to take whatever informed risks, and just move on from such idea instead.