fun ageDescription(age: Int): String {
    return when {
        age / 10 % 10 == 1 -> "$age лет"
        age % 10 == 1 -> "$age год"
        age / 10 % 10 == 9 -> "$age лет"
        else -> "$age года"
    }
}

@Test
fun ageDescription() {
    assertEquals("1 год", ageDescription(1))
    assertEquals("21 год", ageDescription(21))
    assertEquals("132 года", ageDescription(132))
    assertEquals("12 лет", ageDescription(12))
    assertEquals("111 лет", ageDescription(111))
    assertEquals("199 лет", ageDescription(199))
}

public static void main(String[] args) {
    for (int i = 1; i <= 200; i++) {
        System.out.println(ageDescription(i));
    }
}

private static String ageDescription(int age) {
    if (age < 1) return null;
    int lastDigit = age % 10;
    String ageStr = "";
    if (age - lastDigit == 10) {
        ageStr = " лет"; // 10 - 19 лет
    } else if (lastDigit == 0) {
        ageStr = " лет";
    } else if (lastDigit == 1) {
        ageStr = " год";
    } else if (lastDigit <= 4) {
        ageStr = " года";
    } else {
        ageStr = " лет";
    }
    return age + ageStr;
}