I suppose that the age you start Test Cricket will vary a lot depending on which country and how well they are doing.

If your country isn't doing too well, you may fell that you want to change some players. You can look at players you've rejected in the past or you can try somebody younger. For the Aussies it's been hard to get into the team because the established players have been doing so well. And if a player has a drop in form there's no need to rush him out of the side if his team-mates are still performing well and the side is winning. So when a new player comes in he has plenty of of experience and his skills are well developed.

But then sometimes some schoolkid comes along and amazes everyone with his talent. Sachin Tendulkar, anyone?