"He is eleven years old."
Translation:Ему одиннадцать лет.
Literally: eleven summers [years] (are) to him / for him / in favour of him.
That is a 'special' structure in some European languages with declensions.
For example, Latin also uses the structure: sunt mihi multi amici (I have many friends, lit. (they) are + to me / for me + many friends).
For reference, лет (summer) is an irregular plural form of год (year). The numbers 1, 2, 3, 4, 21, 22, 23, 24, ... require singular nouns after them (in nominative or genitive), but 5, 6, 7..., 20, 25, 26, ... require plural genitive.
It is quite interesting to compare the irregular plural forms of the noun 'year' in different languages. In Russia (50--60N), where it is usually cold and harsh in winter, the best time of a year will be summer. In China, the climate is somehow mild, so Mandarin (spoken around 40N) refers the years as springs and autumns. Meanwhile some southern Chinese dialects refer the years as winters, because these areas (20--25N) are subtropical, and in winters it seems a little cooler and agreeable.