More of a Canadian answer - but criminal charges can apply to children, albeit differently. This answer focuses more on Canadian law, especially since this will vary by jurisdiction.
In Canada, prosecution of criminal acts committed by youth are 'regulated' under the Youth Criminal Justice Act. As noted in the Preamble of the act, the justice system recognizes that youth do not have the adequate mental capacity to be completely responsible for their actions, and that rehabilitation of the youth should be performed with involvement in the community, largely through extrajudicial measures (i.e. incarceration and custody is highly discouraged).
Therefore, youth are treated vastly different in the justice system. As for ages, §13 of the Criminal Code of Canada states that no person shall be convicted of an offence when that person is under the age of twelve. The Youth Criminal Justice Act covers youth between 12 to 18.
Most youth caught in the act of committing a crime may not be arrested and charged immediately. Often enough, the police will issue an informal warning or formal police cautions or referral to a community organization. If the police refers the case to the Crown, they may issue a Crown caution, or an extrajudicial sanction - often where the youth will affirm not to indulge in a certain behaviour. This is somewhat similar to a plea bargain - except for that charges can be laid if the sanction is not followed, and agreement to comply with the sanction does not actually constitute a plea.
So, suppose a youth committed a crime, and is subsequently found guilty. What happens?
First off, the trial will be held in a youth court. This is a special court for trying offences by youth, and is completely separate from other adult courts.
Since one of the the aims of the YCJA is for youth to understand the damage done to the community, there may be opportunities for victims to provide a victim-impact statement. These statements can come from the victims of the crime, first responders, and members of the community.
Decision to impose an adult sentence.
If a youth is found guilty of a presumptive offence - which includes murder, attempted murder, manslaughter, and aggravated sexual assault - an adult sentence may be imposed. However, there are limitations to this. First off, they must be of age. The age in which an adult sentence can be imposed varies by province. In British Columbia, this age is 14. In Quebec, it is 16. An offence may also be considered presumptive if it is violent, or if it is a repeat offence.
Of further note is that to impose an adult sentence, it must be shown that youth sentencing options can not rehabilitate the youth and provide a meaningful consequence. The Crown must also ask the court for an adult sentence.
Sentencing is perhaps the most notable aspect for youth crimes. §3 and 4 of the Youth Criminal Justice Act state that measure against young persons must bring a sense of accountability, enforce societal values, and encourage repair of damage done to the community. §5 captures this brilliantly:
5 Extrajudicial measures should be designed to
(a) provide an effective and timely response to offending behaviour outside the bounds of judicial measures;
(b) encourage young persons to acknowledge and repair the harm caused to the victim and the community;
(c) encourage families of young persons — including extended families where appropriate — and the community to become involved in the design and implementation of those measures;
(d) provide an opportunity for victims to participate in decisions related to the measures selected and to receive reparation; and
(e) respect the rights and freedoms of young persons and be proportionate to the seriousness of the offence.
As extrajudicial measures are encouraged, the rates of incarceration are significantly low. Custodial measures are only imposed where youth have committed violent offences, non-compliance with non-custodial measures, and others. Custodial measures often have a maximum of 2 to 10 years, based on the severity and type of the offence committed, as well as any aggravating or mitigating factors. There are also other documents such as pre-sentence reports to consider as well.
Often, extrajudicial measures imposed on youth include community service, restitution, participation in a community organization, and more.