  We present new K- and L'-band imaging of a representative sample of members
of the young 3-5 Myr old sigma Orionis cluster. We identified objects with
(K-L') excess by analysing colour-colour diagrams and comparing the
observations with empirical main-sequence colours. The derived disk frequency
depends on the method used: (54 +/- 15)% if measured directly from the JHKL'
colour-colour diagram; or (46 +/-14)% if excesses are computed with respect to
predicted photospheric colours (according to the objects spectral types,
2-sigma excess detections). We compare the (K-L') excess with other indicators
and show that this is a robust and reliable disk indicator. We also compare the
derived disk frequency with similarly aged clusters and discuss possible
implications for disk lifetimes. The computed age of the sigma Ori cluster is
very important: a cluster age of 3 Myr would support the overall disk lifetime
of 6 Myr proposed in the literature, while an age > 4 Myr would point to a
slower disk destruction rate.
