A new report shows 375 children and teenagers referred to specialist services which deal with a range of problems including anorexia had gone 26 weeks without an appointment.
They are among 5370 young people who were on waiting lists for mental health services across Scotland at the end of September.
Some 94% of children are being seen within the six-month target time frame.
However, it appears adults struggling with drug or alcohol problems are seen more rapidly.
In July-September 2013, 96.6% of the 12,064 people who started their first drug or alcohol treatment had waited three weeks or less, according to the latest report.
Scottish Labour's health spokesman Neil Findlay said the level of delay faced by some young patients was unacceptable.
He said: "Nothing is more important to parents than the health of their child.
"When a problem arises with a youngster's wellbeing it is an incredibly stressful time for the whole family so to have to wait over six months for an appointment just to see a specialist, not even to begin treatment is really not acceptable and more must be done to ensure every child, no matter what their health problem is seen as quickly as possible."
The report, published among a raft of other health statistics yesterday, also found under-18s who need to be admitted to hospital for treatment with mental health problems were often sent to adult wards.
In the first three months of 2013, 55 were admitted to adult wards and 52 to wards for children and youths.
Sickness absence rates for service staff were also reported yesterday. This varied from 8% of hours lost from child and adolescent mental health service staff in NHS Ayrshire and Arran to 2.7% in Lanarkshire.
A spokesman from the Scottish Government said: "Over the past few years we have seen a large increase in access to and reduced waiting times for children needing mental health services.
"Thanks to the hard work of mental health professionals and a £13.5m investment in specialist children and adolescent mental health services Scotland is surpassing the 90% referral to treatment target with an average wait of six weeks.
"The waiting time is measured to the point where the treatment starts. There is a tolerance in the target to allow, for example, when a long assessment is required due to complexity of a child's needs. However, there are still children and adolescents that are waiting too long. From this time next year we will ask boards to achieve the same target within 18 weeks."