Numbers with word cases

cases.cs

public static string Case(
    this int val,
    string one,
    string two,
    string five) {
    var t = (val % 100 > 20) ? val % 10 : val % 20;
    switch (t) {
        case 1:
            return one;
        case 2:
        case 3:
        case 4:
            return two;
        default:
            return five;
    }
}

public static string CaseWithValue(
    this int val,
    string one,
    string two,
    string five) {
    return string.Format("{0} {1}", val, val.Case(one, two, five));
}

public static int Age(this DateTime from) {
    var now = DateTime.Now;
    return now.Year - from.Year -
        (now.Month < from.Month ? 1 :
         now.Month == from.Month && now.Day < from.Day ? 1 : 0);
}

birthday.Value.Age().CaseWithValue("год", "года", "лет");

// 16 лет
// 23 года
// 21 год