What does it mean to be a mature Christian?

We seem to classify those who call themselves Christians under various categories:
new Christians - those who have just come to the faith
mature Christians - those who have been in the faith for a while
nominal Christians - those who say they are Christians but don't seem to have active involvement

Firstly, by labelling people like that can be unhelpful as we are putting them into a box defined by a cultural stereotype. And secondly, didn't Jesus say not to judge so who are we to label others, after then we will be held to the same standard?

Back to the question. So what does it mean to be a mature Christian? I don't think I'm entirely sure the unwritten rules actually help here. It seems to assume that maturity comes after some time of being in the faith and having a somewhat active Christian life (else one be labelled nominal). However, I think that being in the faith for some time and having an active Christian life does not necessarily equal maturity. It also depends on a person's exposure to theology, exposure to good Bible exposition which can help them form their Christian worldview and values and priciples in which to live by.

I have encountered people who have been Christians for a while and have a shocking interpretation of Scripture and prescriptive way of doing bible study (reminded me of being in school again). And also recently encountered someone who though has been a Christian for just over a year, is able to lead bible study like someone who has been a Christian for ages.

As we all know (and have met), there are people who despite their actual age seems to behave like someone much younger than them. This we call immaturity. Yet maturity (a primary criteria being how long they have been a Christian for) seems to be a criteria for some Christian organisations or churches in choosing their leaders (I don't recall ever hearing the term 'immature Christian' used!).

So what is a mature Christian? I'm not entirely sure. And although I don't really like this term, I find that I do use it when I come across new students as I attempt to categorise them so as to spot the up and coming leaders. What are the initial markers? Its listening out (in conversation) whether they go to church, what their view is on certain things, their previous involvement in the Christian world, their current involvement etc. Pretty superficial I would say. But what is the alternative? Maybe next time I should give them some tasks to do - lead a bible study, interact with non-Christians, express their views on certian topics?

But perhaps its more healthy to try not to categorise people (rather unhelpfully) like that (or not categorise people at all if possible). Instead I'll seek to listen to their story. And give people (my students especially) a chance to do what they would like to do or encourage them to continually step out beyond their current comfort zone. Not sure if this is building 'maturity', but then I don't want to be into building mature Christians per se. I'm into holistic faith development and encouraging people (in particular the students I work with) to step out into their potential. Maybe its all one and the same, except that the term 'maturity' is so last millenium.