This is such an interesting thread. I think it's important to be open to what your child tells you at any age. As a PP said, sexual identity is multilayered and people, especially young people, find different layers and identities at different times on the way to finding their true selves. Try not to get too hung up on labelling (It can be hard not to, but try!), and accept however your child identifies at a given time. They may identify differently later, or it may stay the same. Either way is normal.
It can also take people a while to come out to themselves. For me personally, from an extremely young age, maybe 3 or 4?, I preferred females. Later this turned into an aesthetic preference and eventually sexual attraction. But growing up in middle America in the 80s and 90s, I didn't have a name for what I was feeling. I had never heard of gay or lesbian. It was very obvious to me what I was feeling "I like girls. Girls are so pretty. I like to look at pretty girls." While I never had an epiphany that this was "abnormal" (in my culture at the time), obviously I twigged to something because I kept my mouth shut about it and never breathed a word to anyone. When I was older and found out what a lesbian was there was kind of an "Uh-oh" moment and a lot of denial. A lot. Eventually I could not deny it anymore and had to admit to myself that however difficult that path was, I could not be me if I didn't go down it.
If you suspect your child may be GLBTQ, realize that they may need to come out to themselves first. Even if they have a loving and tolerant environment, it can be incredibly difficult to deal with the realization. You may feel like they are shutting you out or keeping something from you, like you have failed and they don't trust you, but in actual fact they could be just starting to come to terms with it themselves. After they have accepted themselves they may feel like coming out to others. Keep the lines of communication open, let them know that you are there if they need you, and let them make their journey.