The English word “year(s)” is expressed in Russian by two words – год (the ordinary world for “year”) and лет (genetive plural of лето = summer).
In Russian, the person whose age is being told is in the dative case. The same rule is applied to objects and things. The age is followed by a word for “year(s)”.
To express age the following rules must be known:
1) After оди́н whether by itself or in a compound numeral, use год for singular and plural forms:
2) After два, три, четы́ре, or compound numerals ending with any of these, use го́да (gen. sing.):
3) From numeral пять, upwards, use лет:
4) After ско́лько, не́сколько,мно́го,ма́ло, use лет
In Russian you can ask about someone’s age two ways. If the person you ask is your friend or someone you address as “ты” say:
Сколько тебе (dat. sing.) лет?
When you ask a person you don’t know or someone who is older say:
Сколько Вам (dat. sing.) лет?
When you don’t know, forgot, or misheard someone’s name you can say:
Тебе (dat. sing.) двадцать лет?