When I realized that you could actually get paid for shouting.
When I was as young as 4 years old, watching old movie musicals with my mom. She says singing and dancing is all I have ever wanted to do for as long as she can remember. I can't imagine doing anything else, and I feel so lucky getting to do what I love for a living!!
Singing "Dentist!" in my high-school run of Little Shop of Horrors.