Etiologies and the whole picture in childhood mental, behavioral, and neurodevelopmental disorders related to gestational age are unclear. This study included all Finnish children (N = 341,632) born between January 1, 2001, and December 31, 2006, whose data including their mothers (N = 241,284) were collected from national registers. Children with unclear gestational age (GA) (N = 1245), severe congenital malformations (N = 11,746), and moderate/severe/undefined cognitive impairment (N = 1140), and those who died during the perinatal period (N = 599) were excluded. The main outcome was the prevalence of mental and behavioral disorders (International Classification of Disorders) at 0 − 12 years of age in association with GA, adjusted for gender and prenatal variables. Out of all included (N = 326,902) children 16.6% (N = 54,270) were diagnosed to have any mental health disorder at 0 − 12 years. Adjusted Odd Ratio (OR) were for any disorder in preterm (< 37 weeks) 1.37 [1.28 − 1.46] and 4.03 [3.08 − 5.26] in extreme preterm (≤ 28 weeks) versus term born children, p < 0.05. The lower the GA at birth, the higher the risk for multiple disorders and earlier onset of disorder, p < 0.05. Adjusted ORs were for male/female 1.94 [1.90 − 1.99], maternal mental health disorder (yes/not) 1.99 [1.92 − 2.07], and smoking during pregnancy (yes/not) 1.58 [1.54 − 1.62], and these risks were more common in preterm versus term born children (p < 0.05). Extreme early birth was a strong risk factor per se for any or multiple and early shown mental health disorders. Other risk factors for mental health accumulated to preterm children.