In his Notes on Vedic noun-inflexion 31 (1942), F. B. J. Kuiper has recognized in Vedic
yu, Av. āyū ‘life, age’ a Proto-Indo-European ∗Aóyu parallel in structure to the ∗dóru, ∗ĝónu, and ∗sónu of Skt. d
ru, Gk. dóru ‘wood’, Skt. j
nu, Gk. gónu ‘knee’, and Skt. s
nu ‘back’. The initial laryngeal has disappeared without trace in Indo-Iranian, even in the oblique cases (e.g. Avestan gen. yaoš from ∗Ayéws), but is indicated by cognates in European languages: Gk. ai
n ‘life, age', aiweí, aién, aiés, etc. ‘always’, and Lat. aevus, aevum ‘age, eternity’, all pointing to ∗Aeyw-, with a vocalism comparable to that of Welsh derw ‘oaks’ and Ru. dérevo ‘tree’. Indecisive in this respect are Germanic cognates such as Gothic aiws∗ ‘age, eternity’ and ajukduþs∗ ‘eternity’. Celtic forms such as W oes ‘age, lifetime’, oed ‘age’ and OIr. aís ‘age; people’ seem to lack ∗w and are not certainly cognate with the family of Skt.
yu and Lat. aevum (cf. Pedersen, VKG 1.176, Walde-Pokorny 1.7, Pokorny IEW 11, and Vendryes, Lexique étymologique de l'irlandais ancien, A 21).