"According to the Omaha World-Herald, the parents of the 8-year-old say the child they first named Ben exhibited girlish tendencies as young as 2, when he would create long hair using scarves. A year later, he donned a tiara and dressed as a princess for Halloween."
If he's been displaying and acting in the gender role of a girl since he was 2, then I'd be safe to say that it's good to start at 8. It's not only enough time (6 years) and he's never acted to the male gender role but it'll increase the chances he'll actually look like a girl. I'm really all for it.