As children grow up it is natural to be curious about the same sex. There is a time in a child's life when the opposite sex seems weird. It is normal to feel safe with children of the same gender. Some times these children will experiment or find each other attractive. This does not make a person gay. It makes a person curious. Only the child can know for certain and sometimes they are not even positive for many, many years.
Granted I have no idea how every individual works but I can safely say that for the most part it's not an experiment. Some people are bi-curious and go forth and get intimate with the same sex or think about doing sexual things with the same sex but most of the time it's absolutely not a phase. Granted I thought it was a phase with me back when I was about 10 and now I'm almost 20 and am fully aware that this is me and I love it! The topic of sexual attraction is a touchy subject but all in all I don't really believe in labels and I don't believe for one second that any of it actually matters. Who you want to get with is simply who you want to get with and anyone who is against it for idiotic reasons isn't worth associating with.
No, multiple studies conclude that sexual orientation is a genetic predisposition. It is certainly not a passing phase. Only the most scientifically illiterate believe that sexual orientation is a chosen lifestyle. If same-sex attraction was a passing phase of youth, there would be little or no older adults in the LGBT community. Sexuality is not a phase, it's a part of who you are.