We present new K- and L ^ { \prime } -band imaging of a representative sample of members of the young 3 - 5 Myr old \sigma Orionis cluster .
We identified objects with ( K - L ^ { \prime } ) 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 \pm 15 ) % if measured directly from the JHKL ^ { \prime } colour-colour diagram ; or ( 46 \pm 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 ^ { \prime } ) 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 .