Is it okay to know your sexuality at 13?

I KNOW I am a lesbian, and I am 13. People always tell me im too young to know, or its just a teen phase. But how come every other kid I know is allowed to say they are straight? Everyone congratulates you when your a 13 year old girl and you have a boyfriend. But If you say you want a girlfriend, everyone looks down upon you and says you don't know what you want? So, can you really know what you are at 13?