We present a new method to derive the age of young (<0.7 Gyr) L dwarfs based on their near-infrared photometry, colours and distances. The method is based on samples of L dwarfs belonging to the Upper Sco association (5 Myr), the Alpha Per (85 Myr) and Pleiades (125 Myr) clusters, and the Ursa Major (400 Myr) and Hyades (625 Myr) moving groups. We apply our method to a number of interesting objects in the literature, including a known L dwarf binary, L dwarf companions and spectroscopic members of the young σ Orionis cluster. Based on observations made with the United Kingdom Infrared Telescope, operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research Council. E-mail: rfj [at] star.le.ac.uk (rfj[at]star[dot]le[dot]ac[dot]uk)