Hi there.It is the age when he committed the alleged assault which will be relevant to the charge, however that does not necessarily mean that his case will be heard by the children's court. This fact sheet has some information that might be helpful:http://www.yac.net.au/childrens-court-queensland/. It also has a number of support organisation that you can get in contact with for further information, such as Legal Aid or Community Legal Centres specialising in helping young people. A lawyer with experience in criminal law in Queensland would be able to tell you want the standard practice was.

Under Australia’s migration law, a visa can be cancelled if there is reason to believe the holder does not pass the “character test” at any stage during their stay. The character test is based on a set of criteria from the Migration Act 1958 (Cth). The Department of Immigration and Border Protection takes these criteria into account when making a determination on somebody’s character. These considerations apply equally to adults as to children and include:

• Whether, having regard to the non-citizen's past and present criminal conduct and general conduct the non-citizen is not of good character.
• Whether the person has a substantial criminal record.
• Whether the person either has, or has had, an association with an individual, group or organisation suspected of having been, or being, involved in criminal conduct.
• Whether there is a significant risk that the person will engage in criminal conduct in Australia, harass, molest, intimidate or stalk another person in Australia, vilify a segment of the Australian community, or incite discord in the Australian community or in a segment of that community.

If the police are successful in convicting the visa holder of an assault charge, this may go to a determination about his character. Legal advice from a lawyer who specialises in Migration law may assist you figure out what the risks are in your particular case, and how to mitigate them.