Of course all of us are always growing up - at least physically (unless one has some kind of disease that's hampering their physical or mental growth). The question here is about our spiritual growth.
If we are not regularly questioning our belief system (not to be confused with a lack of self belief) and finding newer pastures that challenges our existing beliefs with good reason, our growth is stunted.
For years, we tend to believe in something - an opinion that we formed due to peer pressure, familial roots, childhood and adolescent experiences, or simply because someone convinced us to believe in something and we were too lazy or indifferent to think for ourselves (at least at that moment).
If we sit down and ponder, a lot of our beliefs are not exactly our own, and chances are that the ones that we did form ourselves are the product of inexperience or formed sometime in the past at a point where our reasoning was affected by emotions.
So how do we grow up and go about challenging these beliefs? It's not a simple process and it definitely requires a lot of slow thinking and a well thought out framework.
The simplest framework that we can use is to put forward our 'Arguments For' and 'Arguments Against' for that belief; and to understand the time of point in our lives when we actually formed those opinions and beliefs to get a hang on the point of reference that we used while we worked on those beliefs.
It is not an easy task and we may be surprised at how often we were holding onto an ideology that either isn't ours, or, was plain crap. Our beliefs tend to change as we age and mature, and hence, this should be an on-going process, throughout life.
We would know we are truly growing up when we start to realize that our beliefs are no longer a by-product of flawed reasoning, altered emotional state, an idea formed by others (not that it is an entirely wrong way to go about it), and brings us more confidence and self-awareness in the process.