I think part of growing up is learning to accept who you are. If you look at younger people, it's quite easy to put them in groups (which no doubt have different names in different countries- I won't give examples because I don't want to confuse people!) Children all dress in pretty much the same way (because their parents still dress them and want them to look cute and innocent) and they all enjoy most of the same things - going to the park, colouring in, playing with dolls/action men. When we get a little bit older then we rebel against our parents and attempt to find our own identity, but because we're still so young then (say 13 or 14) we don't really know what is truly 'us', so we dress the way our friends do and like (or pretend to like) the same things.
Once we reach maturity it's so much more difficult to put us in boxes - we're all so different, with different interests/hobbies, and we all like different things and dress in different ways. Once we've reached the time in our life when we truly know who we are, we also tend not to have the same insecurities that teenagers do - it's so much easier to say 'take me or leave me, but this is who I am.'
So yes, being authentic does come easily to me because I know who I am, my friends and family love me for who I am, and if others don't like it then I'm really okay with that. That hasn't always been the case, when I was younger I wanted to fit in just as much as everyone else, but now I'm all grown up and comfortable with who I truly am :)