But turning older doesn't make you feel any older. It's just a number. You can still be the same young soul on the inside.
At least, that's what you think - until you talk to people who are half your age and you realise that you have, in fact, grown up a bit.
I was reminded of this when I travelled back in time to give my younger self some advice. He was in his second year at university, tapping out a literature essay on an electric typewriter.
"I am your older self and I am visiting you from the future with my collected years of wisdom," I announced.
My younger self handled it really well. "Nice beard. Mate, I can't believe I'm going to grow a beard when I'm older."
"Thanks. Now look, I'm here to tell you some things that will help you through the next 20 years."
"How long did it take to grow? Mate, I tried to grow a goatee but it went all fluffy."
"It took a couple of weeks. But listen, I need to tell you ..."
"Am I married in the future? Am I rich? Where do I live? Will they end up making more Star Wars movies? Mate, I bet they're totally awesome."
And so on. He kept saying "mate", which was kind of annoying. He also talked too much. This kid was way too hyped and he hadn't even discovered coffee yet. It made me realise I like myself a lot more now that I'm 40.
For starters, I've learned to be a better listener. Twenty years ago, I was dead certain about my own opinions. These days, I'm more cautious of absolutes.
I'm also a lot more comfortable in my own skin. I used to wear my confidence as a disguise to hide the fact that I usually felt like the smallest, most vulnerable guy in the room. Thinking about that, I realised my younger self's blustery chatterboxing was probably his way of coping with an intimidating situation. After all, right now he was suddenly faced with an older, more self-assured version of himself.
"Relax," I said to him. "You're over-compensating. You don't need to do that with me. In fact, you don't need to do that with anyone."
"It's the beard," he replied. "It makes you look more grown up."