The aim was to explore the predictive value of objective treatment need, impaired quality of life, and parental influence on orthodontic treatment demand in preadolescents and adolescents. A secondary goal was to validate 16-item Child Perceptions Questionnaire for the 11-14 year age group (CPQ11-14): item-impact and stepwise-regression short-forms. A convenience sample of 287 participants (55% female) was used for validation. Internal consistency, test-retest reliability, and convergent validity were evaluated. Predictors of treatment demand were explored in 197 orthodontic patients: 93 preadolescents (51% females), 104 adolescents (55% females). Hierarchical linear regression and multilevel logistic regression models were used to explore the predictive power of age, gender, objective treatment need, impaired quality of life and parental influence on treatment demand. CPQ11-14 regression short-form had acceptable psychometric properties. Significant linear predictors of treatment demand were impaired emotional well-being (EW) (β = 0.335, p = 0.002), parental influence (β = 0.221, p = 0.002), and malocclusion severity (β = 0.152, p = 0.025). In logistic regression, parental influence was revealed as the most important predictor of treatment demand in preadolescents, OR = 7.7 (95% confidence interval CI 2.4-25.1; p = 0.001); objective treatment need in adolescents, OR = 4.5 (95% CI 1.5-12.9; p = 0.006). The increase of impairment in EW by one scalar point increased treatment demand by 1.4 (95% CI 1.1-1.9; p = 0.017) in preadolescents; 1.3 (95% CI 1.0-1.7; p = 0.021) in adolescents. Greater parental agreement and motivation for treatment could result in higher preadolescents' cooperation. Orthodontic treatment in adolescents might be more effective with the patient-oriented approach.