Although several cross-sectional age-related studies of harmonic perception in children have been performed, studies of harmonisation are very few. In the present study, the ability of school-aged children and adolescents to add chords to an ongoing tonal melody is investigated. Age-related development of harmonic features, chord rhythm and types of harmonisation are analysed. Six to fifteen-year-old participants (N=44) harmonised a C-major tonal melody. Four keys of the synthesiser, each producing a whole major triad (C, D, F and G), were used in real-time chord production. The frequency of chord I as the final chord, distribution of chords in strong metrical positions and number of periodically harmonised measures increased with age. The number of events and chord transitions and frequency of out-of-key chords decreased with age. At the earliest level of development children focused on rhythm or metre, producing a high number of events. At next level, coordination of melody and harmony improved, and tonic closure became common. Chords emerged at strong beats or formed a harmonic 'riff'. In the most developed products, melody and chords were well-coordinated, the most frequent chord being I, tonal closure common and the majority of chords emerged at the strongest beat.