The development of audiovisual facilitation of motor responses to concurrent audiovisual stimuli in school age children has not been previously reported. Thus, this study investigated multisensory integration in late childhood and early adolescence at both a behavioural and neural level. Motor reaction times (MRTs), accuracy and high-density electroencephalography (64 scalp electrodes, sampling rate: 488 Hz, filter bandpass: 2.6 – 100 Hz at 12 dB/Oct) were recorded while children, adolescents and adults performed an audiovisual detection task. Stimuli included auditory (AT), visual (VT), audiovisual (ATVT) targets and blank invalid trials presented in a random order for the duration of 40 ms. All 3 age groups were faster at detecting the ATVT stimulus compared to AT and VT stimuli. Cumulative density functions (CDFs) of MRTs demonstrated that discrepancies between ATVT CDFs and AT+VT CDFs were greater for adults than for children or adolescents. Multisensory integration at a neural level was isolated by subtracting the sum of the two unisensory evoked potential from the multisensory evoked potential (ATVT–[AT+VT]). Multisensory integration observed at 100 ms post stimulus onset was distributed over occipital-parietal electrode sites in children and adolescents and more localised to occipital electrode sites in adults. Both behavioural and electrophysiological measures suggest that multisensory integration is immature beyond late childhood and early adolescence. Dissimilarity in topographic distribution of multisensory neural processes in children, adolescents and adults may be associated with differences in cortical myelination and organization, which continue to mature until early adulthood.