Age 4 is generaly the age to start looking for representation in acting, but do realize that some “talent” agencies have a print modeling division to! Which means that they may take children under 4 in that agency for modeling. It’s best to check-out their website or call the receptionist for a complete list of divisions before you start sending in your pictures.
Gilla Roos is a reputable model and talent agency in New York City where you can submit pictures through their website (but just make sure you live close enough to the city to go on appointments!): www.gillaroos.com
And LA Models is a great one in California: www.lamodels.com – but you have to send in your pictures by postal mail.
Good luck! Would love to hear some success stories along the way!!!