If you need any more proof you don't have to grow up, I'm 44- yes, old enough to be your father- and I don't feel a day over 18. As you get older, you will have more responsibilities, more commitments, more obligations, but by no means do you have to leave your childhood behind.
If you're able to hold on to that childlike innocence, that sense of wonder, a sense of optimism, finding joy in the simple things in life, you may grow older but not up.
At my job there are younger people in their twenties who are already "old". In such a short time they've become cynical, jaded, spiritually dead. Some of them have already turned to drugs or alcohol to accelerate their trip to an early grave. But it doesn't have to be like that.
And yes, way back in the dark ages, back when the Atari 2600 was hi-tech, I was 15 once too.