Did you know that we use the noun год (year) with the number one and all the numbers that end in one, that we use года with the numbers from 2 to 4 and all the numbers that end in 2, 3 and 4, and лет with all the numbers from 5 to 20, and all the numbers that end in 5,6,7,8 and 9? So, год is Nominative singular, года is Genitive singular and лет is Genitive plural.
Моей дочери один год. My daughter is one year old.
Ему только три года. He is only three years old.
А моему папе 56 (пятьдесят шесть) лет. And my Dad is 56 years old.