What age do mental health conditions develop at? “We know that 50% of mental health problems develop before the age of 14 and that 75% develop before the age of 18." Barbara Keeley MP, 8 March 2018"75% of those with a mental health condition start developing it before the age of 18" MQ: mental health charity“50% of mental health problems are established by age 14 and 75% by age 24.” Mental Health FoundationThere are a number of different claims regarding the age at which most people with a mental health condition begin to develop it. The participants’ mental health was assessed in a number of ways—they were given psychiatric interviews at ages 11, 13, 15, 18, 21 and 26, it was determined whether they were accessing mental health treatment already and if they were using “intensive mental health services”. Although the age at which conditions first appeared in participants varied depending on their mental health condition, when looking at all mental health conditions being examined in the study the researchers found that around 50% of conditions had developed by the time participants were age 14.