New data shows the waiting times target for Child and Adolescent Mental Health Services (CAMHS) has been met for fewer than two-thirds of young patients seeking help.
Liberal Democrat health spokesman Alex Cole-Hamilton said NHS performance in this area had now "plummeted" to the worst on record.
He spoke out as NHS figures revealed at the end of September 2019 there were 10,034 youngsters waiting to be seen - including 602 who had been waiting for 53 weeks or longer.
This compares to the 221 children and young people who had been waiting for a year or more at the end of September 2018.
Campaigners at the Scottish Children's Services Coalition (SCSC) said the figures showed Scotland was "continuing to fail thousands of children and young people with mental health problems, with more clearly needing to be done to address this epidemic".
READ MORE: Number of youngsters waiting for mental health care treblesWhile the Scottish Government has set the target of having 90 per cent of youngsters who need CAMHS help receive this within 18 weeks, over the period July to September 2019 this target was met for just 64.5 per cent of patients - down from 69.7 per cent in the previous quarter
The latest data showed just two mainland health boards - NHS Borders and NHS Dumfries and Galloway - met the 90 per cent target
Meanwhile, in the NHS Grampian area, only 50.8 per cent of youngsters who started treatment in July to September were seen within 18 weeks.
Mr Cole-Hamilton said: "Just last week, the Government denied there was a mental health crisis.
"Today, the length of time children and young people are waiting for treatment is the worst on record. Performance has plummeted.
"The SNP Government is failing a generation of young people and the consequences of their waiting up to two years for treatment are heartbreaking.
"Problems that start small are becoming crises as help arrives too late."
Labour health spokeswoman Monica Lennon said: "These new figures reveal what we already know - on access to mental health services, the SNP are failing Scotland's young people.
"It's just not good enough that in 2019 thousands of young people have waited over four months to be seen by specialist mental health services and hundreds have had to wait over a year.
"At a time when we know youth suicides have been increasing these figures should shame SNP ministers into action."
Alison Johnstone, Scottish Greens parliamentary co-leader, added her concerns, saying: "When young people in distress cannot access treatment when they need it, something has gone terribly wrong.
"The Scottish Government needs to review this situation urgently and ensure services are getting the support they need to meet demand, or Scotland's young people will continue to suffer."
The SCSC spokesman said: "These newly released figures highlight that the NHS in Scotland, including nine of our health boards, are failing to meet what is already a lengthy waiting time.
"Yet we know that three children in every classroom has a clinically diagnosable mental health problem.
"There must be a radical transformation of our mental health services, with a focus on preventing such problems arising in the first place and intervening early, especially when we know that half of all mental health problems begin before the age of 14."
READ MORE: Suicide-risk teenager waits weeks for help as health services struggle to copeMental Health Minister Clare Haughey said: "We want to make sure anyone who has identified as needing support can get services that are appropriate to their needs.
"To shorten waits for treatment, we are making significant changes to meet increasing demand and to ensure everyone gets the right treatment, at the right time and in the right place.
"This includes the rollout of our £250 million package of measures to support positive mental health for all."
She added: "We are also strengthening the support available in communities and schools with mental health first aid training for local authorities, ensuring every secondary school has access to a counselling service by September 2020 and training 250 additional school nurses over the next three years, with 50 already in place this year.
"Since 2007, CAMHS staffing has increased by 74% and in the past year, we have seen an increase of 1.2 per cent - the majority of which was in psychology staff - while we continue to create new posts in this area.
"This year's Programme for Government builds on this progress even further.
"That includes putting in place community well-being services for children and young people aged five to 24 and their families across the whole of Scotland, a new 24/7 crisis support service for children and young people and a new Adult Mental Health Collaborative so public services, the third sector and communities can work closer together to improve support to people suffering from mental ill health."