At what age do kids decide if they are gay?

Kids do not "decide" if they are gay or straight. For most people, awareness of their sexual orientation comes a puberty.

An Additional Perspective:This is a hotly debated issue. While some claim that sexual attraction is a matter of genetics or in uterodevelopment, no amount of scientific research has yet revealed any link at all; only further speculation has resulted.

That being factually stated, all human behavior is a matter of choice (either the actual choice or the refusal to choose--both are choices). People choose their mates, or not. People choose their associations, or not. People choose their form of sexual expression. Consciously or unconsciously, it is all still a matter of choice. So, people choose to be heterosexual, homosexual, or asexual, as least at to their manner of practice.

Sexual orientation is typically a question answered at puberty when sexual attraction is nascent, but social-cognitive theory dictates that environment (nurture in what ever form it is experienced) can play a huge role in how and when that particular decision is made.