The phenomenon of ageing of living creatures may be studied in the context of hierarchic thermodynamics. Ageing (ontogenesis) is regarded as a process of formation of the body's submolecular structure. A concept of the degree of ageing (the completion of ontogenesis) which is evaluated from changes in the Gibbs specific function of formation of the submolecular structure of the body's biological tissues is introduced. Quantitative criteria are proposed to find differences between the chronological and thermodynamic age of biological objects. The experimentally based thermodynamic approach reveals the impact of environmental parameters on longevity. The new discipline thermodynamic gerontology which will, in terms of the physical theory, substantially affect human longevity may be hoped to be formed in the coming years.