It's a damn good question. Too often, people think their own particularly cheesy way to view things sets the standard for maturity.
I view maturity as the ability to be real with yourself and others about your thoughts, feelings and views, to own them as your own, and to make good on your insights while thriving in this mode of living. Good judgment helps.
Blathering on and on about your unsupported pet hunches is not maturity. Abdicating the use of reason is not maturity. Lying to yourself and placating people to the detriment of your integrity is not maturity. Nor is sycophancy.