You are mature when:

You pick your fights.
You realise that the world isn't centred around you!
You don't hate people for no apparent reason.
You can forgive people, and yourself - nobody is perfect.
You learn and rise from your mistakes and failures.
You can keep calm and act sanely in stressful situations.
You realise that age isn't so important after all.
You realise that life is short and you grow older - faster each year.
You can put other's needs before your own.
You can schedule your time, and are able to make long term commitments,- and keep them!
You can foresee the consequences of your actions, and you take responsibility for them.
You realise that getting wasted, doing drugs, etc. isn't "cool".
You respect other people for who they are.
You stop conflicts instead of escalating them.
You are someone whom somebody could genuinely trust.
You understand the importance of money, and take responsibility for your finances.
You don't judge people by what they cannot but rather by what they can.
You don't need to prove how much better than everybody else you are.
You don't have to show off that you're using the latest Iphone, etc.
You can adjust your language to the receiver, so you don't overcomplicate things just to sound smarter.
You drop the "I don't care" attitude.
You stop updating your Facebook profile picture - weekly!
You keep the selfies taken down to a minimum.
You are capable of sending a message and update your status - all while using less than 50 emoticons!
You realise; you don't have to look like a model in order to be beautiful!
Guys: When you see that women are not just objects for you to toy with!

Maturity is a lot of things, the list could go on forever, but here are some things maturity definitely is not!

Maturity is not:

When you hit puberty.
When have your first boy/girl-friend (somebody should make a unisex term for that).
When you get your first job.
When you have sex for the first time.
When you are of age.
When you are no longer in school.
When you get married.
When you get kids.

Some never mature!

Maturity is not something you become when you hit a certain age or accomplish some achievement. Rather, maturity is about living and acting a certain way, and is very much about respecting, and getting along with others, while having a degree of humility.

This is my subjective view of what maturity is, and is purely based on my, admittedly limited, experience. - Feel free to disagree with any of the points above.

Source -quora.com