It's not something that comes with age. It's not something that is relative to maturity. Being genuine is a moral thing.
That being said, there is a certain amount of responsibility in the way you live your life, whether you like it or not. Every decision has a consequence, and no decision you make can be blamed on anyone else.
People have a way of avoiding responsibility, and in doing so, making excuses for their lack of genuineness. "I didn't tell you about ___, because _______." "I didn't ask you to _____, because so-and-so said______." "It;s not my fault, it's your fault, because ______."
Stop it.
You are granted almost complete freedom in your life, you need to learn how to be responsible for that freedom. Sure, living in a society restrains certain freedoms. If you don't like the social constraints, leave your society. Back to reality:
You are free, therefore you must accept the responsibility of your life. Lying was not brought on by someone else. You chose to lie, you deal with the consequences. You choose to cheat on your partner, you wear the guilt of it. If you choose to hurt a loved one, You must carry the weight of that betrayal.
I've encountered many forms of this in the past month or so specifically. This has mainly been in cross-gender friendships I'm involved in, which should not come as a surprise, considering they can often be complex situations.
It can be as simple as a clever lie to parry a curious question I asked (which, evidently, was not so clever). It can be as big as inventing a reason to not hang out. It can be as complicated as mixing fact and fiction to describe a disagreeable recent-past event as "for the best," when in reality, that's not the way the person was truly feeling.
All of these are lies, and there is a serious problem when it comes to misdirection and dishonesty in our day-to-day interactions. Lies compound themselves, and begin breeding more lies. Lies also grow from small, controllable fact-fiction hybrids, into untamed beasts of deceit.
"So what's is your point, Andy?" Well, it's simple: We need to start treating one another better, and living our lives in a more honest and responsible way. We need to practice being open with our communication, and realizing the consequences that our actions have on the people around us.
I understand the counter-arguments. "Not everyone is going to do this." So what? It's better to live your life in a moral, and respectable manner, and your influence may spread beyond yourself and encourage this healthy way of living amongst your friends and family. "If I'm always honest, and the other person isn't, I'll just get hurt." If you're dealing with a snake like that, you're going to get hurt anyways. There's no reason to act like an abusive idiot, just because someone has abused you.
In conclusion, I hope to see more people behaving responsible. If you make a mistake, confess it to the people who are affected. Make amends with them. Error is a human trait, as is forgiveness. Practice both, and we'll all be happier for it.