Disclosed herein is a computer-implemented method of optically determining an estimated age of a skin of a living being. The method includes: i. using at least one sample reflection spectrum of at least one portion of the skin of the living being over a spectral measurement range, the spectral measurement range including at least one portion of the wavelength range of from 1 μm to 2.5 μm; and ii. determining the estimated age of the skin of the living being by applying, to the sample reflection spectrum, at least one trained trainable model. The trainable model is trained on a training dataset including a plurality of labeled reference reflection spectra. Each of the reference reflection spectra is acquired over a spectral range at least partially overlapping with the spectral measurement range of the sample reflection spectrum of step i.