For a 20-year old who needs and wants to grow up in the normal societal sense, they must take on more responsibility and become less dependent on others (parents, teachers, friends, siblings) to manage their life and make progress toward adulthood. They need to have a healthy mixture of independence and a sense of responsibility.
For a Christian believer, no matter their age, who wants to Grow Up in a spiritual sense, their need is quite different from practical life maturity. Growing Up for the Christian begins with less dependence on the self and full dependence on God. Responsibility is important but it is you taking responsibility for your Growth and doing whatever is necessary to allow God to bring you to maturity.
My own spiritual breakthrough came from the realization that I had not placed Christ at the center of my life. I had not made him Lord of my life. I had to lose the grip I had on managing my own life and trust that Christ’s ways were infinitely better than my own ways. I had to lose my independence (self will) to Christ in order to gain independence back.
You see, now, under the Lordship of Christ, I am no longer under the confines of my own limitations, imagination, and circumstances. Instead, I am an inhabitant of the Kingdom of God with the promises and gifts of Christ at my disposal. That is incredibly freeing.
Do you want to Grow Up?
First, surrender your independence and place Christ at the center of your life. This was the first step for me and I have never regretted it.