Personally I think O&G would be a bad option for a beginner at a young age. The random and silly part of them may be appealing but the player could be heading for a big disappointment due to all the rules and the fact that, well, all the cool silly stuff just doesn't do that well. I suppose O&G could work for starters if you ignore the Animosity rule.
If you are starting at 500pts, can you even make an Ogre army!?
Empire would be a good one to start with as they are the Swiss Army Knife of the Warhammer world - they have a little bit of everything. They don't excel at any one phase but on the reverse they are competent in them all.
I agree w/ the meister in that you should ask them what appeals to them aesthetically. And second, maybe ask what style of play appeals to them: do they want to be up in the enemy's face in CC? Do they want to stand back and shoot from a distance? Do they want to dominate the magic phase? That last one should probably wait until they get the basics down though.