Originally published by Hannah Ritchie in Our World in Data on October 19, 2026, under a Creative Commons Attribution 4.0 licence. Adapted here under that licence: condensed and restructured, with new key takeaways, a “why it matters” section and an FAQ. The chart in the original draws on Solmi et al.’s 2022 meta-analysis, also published open access. Our World in Data and the author do not endorse TeaLamp. Prepared by the TeaLamp Editorial Team with the help of AI.
When does mental illness first appear? For most people who will ever experience one, the answer is earlier than many expect. A large global analysis found that about a third of people who develop a mental disorder do so by age 14, and two-thirds by age 25. The peak age for a first experience is just fourteen and a half.
Key takeaways on mental illness
- About 34.6% of people who ever develop a mental disorder first experience it before age 14, and 62.5% before age 25.
- The overall peak age of first onset is 14.5 years, and the median is 18.
- Onset timing varies by condition: social anxiety, OCD and eating disorders cluster in the teens; alcohol, cannabis and schizophrenia diagnoses spread through the twenties.
- First symptoms often arrive years before a diagnosis: for depression, the median age of first symptoms was 26, but 31 for diagnosis.
- Most studies behind the data come from high-income countries, and “onset” was defined differently across them.
The largest study of its kind
The numbers come from a 2022 meta-analysis by Marco Solmi and colleagues, published in Molecular Psychiatry, which pooled 192 epidemiological studies covering more than 708,000 people worldwide. It is the biggest synthesis of its kind on when mental disorders begin. Hannah Ritchie of Our World in Data turned its findings into an accessible explainer, sorting nine disorders into three groups by when they tend to arrive.
Three patterns of arrival
Some conditions can begin at almost any point in adult life. Generalized anxiety disorder, depression and bipolar disorder are spread fairly evenly across adulthood, with no single sharp peak.
A second group clusters in early adolescence. Social anxiety is the most striking: nearly 80% of people who develop it do so before age 18. Obsessive-compulsive disorder and eating disorders follow a similar early pattern, with eating disorders peaking around age 16 and a median onset of 18.
A third group peaks later. Alcohol and cannabis use disorders, and schizophrenia, tend to be diagnosed in the twenties, though their curves still stretch across adult life.
Like tree rings, the timing of first symptoms leaves its mark across a lifetime. Image generated by AI.
Symptoms arrive before labels
One of the most useful findings is the gap between first symptoms and first diagnosis. For depression, the median age of first symptoms was 26, while the median age of diagnosis was 31. People often live with symptoms for years before the condition is named.
That gap matters because the data also show that early support helps. The meta-analysis authors write that good mental health promotion, prevention and early intervention before or at the onset of disorders improve outcomes. Knowing the vulnerable windows is the first step to timing support well.
Support should be timed to vulnerability, not to birthdays. Image generated by AI.
Why the current system misses the peak
Most mental-health systems are split into child and adult services at age 18. But the onset data show risk peaking and spreading right across that boundary: the overall peak is 14.5, the median is 18, and a substantial share of first experiences land in the late teens and twenties. As Ritchie puts it, support should be timed to vulnerability, not to birthdays.
What the numbers do not say
The figures describe when conditions begin among people who ever develop them, not how common each condition is. And they come with honest limits. Different studies defined “onset” differently: first symptoms, first diagnosis or first hospitalisation. Many people experience recurring episodes that onset age cannot capture. And most of the underlying studies were done in high-income countries in North America, Europe or Australia, so the picture may look different elsewhere.
Why it matters
Timing changes how we think about support. If a third of disorders begin by 14, then schools, families and youth services are not just background settings — they are the front line. And if symptoms typically precede diagnosis by years, then noticing early signs and making help easy to reach may matter more than any single treatment. The numbers are a reminder that mental health is not an adult problem that appears one day; for most people, it is a young person’s story.
Related reading: Neuroticism and the Pandemic: Why Some Minds Struggled More
Frequently asked questions
Does this mean a third of teenagers have a mental disorder?
No. The figures describe timing, not prevalence. Of the people who will ever experience a disorder at some point in their lives, about a third first experienced it by age 14. It does not mean a third of all 14-year-olds are affected.
Which conditions start earliest?
Social anxiety, OCD and eating disorders cluster earliest, with most cases beginning before or around the late teens. Anxiety, depression and bipolar disorder are spread more evenly across adult life.
Does an early onset mean a worse outcome?
The study does not say that. What it does say is that early support and intervention improve outcomes — which is exactly why knowing the typical windows matters.
Sources
- Hannah Ritchie, “When do people experience mental health conditions for the first time?”, Our World in Data, October 19, 2026
- Solmi, M. et al., “Age at onset of mental disorders worldwide: large-scale meta-analysis of 192 epidemiological studies”, Molecular Psychiatry 27, 281–295 (2022)
- American Psychological Association summary of the National Comorbidity Survey Replication on early onset
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