If someone has a mild depressive episode they have less of the above symptoms and are able to keep going with their daily life. As it develops into moderate depression, more of the symptoms develop and managing normal functioning becomes harder. By the time they reach severe depression, they have most of the symptoms and are really struggling to get on with their day.
I would suggest accessing help from a psychiatrist with any of these severities as treatment can be given to help prevent the symptoms worsening, to lead to recovery and to prevent relapse in the future.
It is also very important to access help if the young person is having episodes where their mood goes very high at times. I have seen young people who have episodes of low mood and then switch to being over excited, they talk too quickly, they may say thoughts race and they might do some reckless things. If this occurs, they need an assessment from a psychiatrist for bipolar disorder.
It is so important to get help for young people with depression or bipolar disorder as suffering with these conditions is at times like being tortured and treatment can prevent a reduction in functioning and academic attainment, prevent them from losing relationships, as well as save lives. Without treatment a small proportion of young people might go on to develop psychotic depression, where they become quite paranoid that people want to harm them, they start hearing voices and they truly believe others would be better off without them.
Depression is treatable but it takes time for medication to take effect and getting treatment sooner rather than later, may prevent the need for medication and certainly avoid considerable suffering and risk.