Hopefully Mango will soon be talking since he has a interest in it
http://youtu.be/0EhgN4lxldU

Actually this is a juvenile cock. Plum head cocks begin to get the vibrant masks around usually around the age of 2.

Actually its a Female lol only Females get a yellow ring the babies don't have that and she doesn't have the red wing patches Males have.

after looking through pictures of juvenile male plum heads, it seems you really can't tell their gender while their young. They don't get the red patches until after they've gone through a juvenile molt.
I found this website that was selling a couple of male juvenile plum heads. They don't look any different from females to me, aside from the one that's facing the camera, which seems to start getting lighter color.
http://www.postadsuk.com/2-beautiful-ba ... 5-100.html

It also seems that adult females too have the yellow ring separating their face from their green body, just like juvenile males.

If you look closely, you can see the yellow around the neck on that bird. It's just not as bright due to the lighting.

So chances are it is a juvenile male, if you also take into consideration that the bird's name is max and the description refers to the bird as a he.

And if you look in the comments, you can see the owner post this:

Oh it would be great if I had Mango as a baby and watched him grow and change colour.Its easy to get Cockatiels as babies and it was great watching Rocko grow up but now my baby is all grown up.