There are six year olds who know very clearly, and there are people just coming to terms at 50 and 60. There's no set age. Whatever your sexual fantasies are... That is usually pretty reliable indicator.
Sexual development can start around 8-9 years old, so it's possible to know as young as that. Some people say they knew their sexuality before puberty. It's also normal for sexual development to still be going on in your late teens, so don't worry if you don't know your sexuality yet. Even then, some people don't figure out their sexuality for decades after puberty mostly for psychological or social reasons.
I found out late in life my true sexuality. I felt perfectly happy as a straight woman until I was about 30 but when I started having feelings towards another woman I was confused. I sought help from a therapist who helped me come to terms with what was a hard time for me. 30 years on I am an older, wiser comfortable lesbian happy in herself. I can't give you an easy answer because only you can answer that question. Whatever you find out about yourself, I hope you are happy with your findings.
