Maturity is not necessarily a good trait in an employee. Companies hire recent grads specifically because they are immature and will be easier to manipulate. And conversely, companies are often wary of older, mature candidates because such employees won't be easy to manipulate and exploit.
Also, mature people usually come to the conclusion that working a job for someone is pretty sucky way to live, and will either come to terms with it, which often leads to trying to get as little done for as much money as possible (i.e. getting a good deal), or will want to go independent, which will make them unhappy while they're employed. These issues are usually non-existent in the graduate crowd, as they are just amazed that someone is treating them as adults, giving them responsibility and paying for completion of tasks.
Anything to do with several possible answers and assessing how they lay out different solutions, their trade off and work towards a pragmatic and incremental solution.
Speaking from the perspective of someone who entered the industry relatively young and had the opportunity to grow into management roles: I'd say the hallmark of maturity is willingness to adapt to change, take necessary risks and overall maintain their individual interests in projects responsibly. Maturity is in fact, just survival. That is, do you think this person can hold their own, choose their tactics and strategy wisely and pick their battles? Passion is good, maturity is nice to have, but I'm consistently surprised by what young and old people can do given the right opportunity, the right management and the right culture.
Also, mature people usually come to the conclusion that working a job for someone is pretty sucky way to live, and will either come to terms with it, which often leads to trying to get as little done for as much money as possible (i.e. getting a good deal), or will want to go independent, which will make them unhappy while they're employed. These issues are usually non-existent in the graduate crowd, as they are just amazed that someone is treating them as adults, giving them responsibility and paying for completion of tasks.