The age calibration of the Washington δT<SUB>1</SUB> index is mainly used to estimate ages of star clusters older than 1 Gyr and no age-metallicity degeneracy effect is considered. We have profusely exploited synthetic T<SUB>1</SUB> versus C - T<SUB>1</SUB> colour magnitude diagrams to explore the intrinsic behaviour of the δT<SUB>1</SUB> index. The analysis shows that δT<SUB>1</SUB> varies with age and metal content as well. In general, the dependence on age weakens for ages greater than ~6 Gyr, and is even less sensitive to age as the metallicity decreases. For ages younger than ~5 Gyr, δT<SUB>1</SUB> shows a strong correlation with both age and metallicity. The δC index - defined as δT<SUB>1</SUB> for the C passband - is also a combined measurement of age and metallicity. We introduce a new age-metallicity diagnostic diagram, δT<SUB>1</SUB> versus δC - δT<SUB>1</SUB>, which can unambiguously provide age and metallicity estimates, simultaneously. The new procedure can be used to derive ages from 1 up to 13 Gyr and metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the cluster reddening and distance modulus. It solves the constraints found in the δT<SUB>1</SUB> index and surpasses the performance of the standard giant branch metallicity method. All these features make the diagnostic diagram a powerful tool for estimating accurate ages as well as metallicities.Facultad de Ciencias Astronómicas y Geofísicas