In Russian you use the dative case when talking about your age. This is why you say:
"Ему три года.'' He is three years old.
''Eй восемь лет.'' She is eight years old.
''Нам двадцать лет.'' We are twenty years old.
It's important to look at the pronoun or name used here. It's always in dative case. Then you change он with ему, она with ей, and so on... These are the Russian pronouns in dative case:
I = мне. You = тебе. He and It = ему. She = ей. We = нам. You (plural) = вам. They = им.
The word ''year/s'' also changes depending on the number you use: If your age ends with 1 you say “год” . If your age ends with 2, 3 or 4 you say “года” . And if your age ends with any other number (0, 5, 6, 7, 8, 9) you say “лет”.
I hope this helps. I will also appreciate if you correct me in case I said something wrong.