永遠の17歳。 Java8でコンパイル・実行してね。

日本人.java

import java.time.OffsetDateTime;
import java.time.format.DateTimeFormatter;

public class 日本人 {
    protected final String name;
    protected final OffsetDateTime birthdate;

    public 日本人(String name, String birthdate) {
        this.name = name;
        // 2011-12-03T00:00:00+09:00
        this.birthdate = OffsetDateTime.parse(birthdate+"T00:00:00+09:00", DateTimeFormatter.ISO_DATE_TIME);
    }

    public int getAge() {
        return getAge(OffsetDateTime.now());
    }

    protected int getAge(OffsetDateTime today) {
        int age = today.getYear() - birthdate.getYear();
        if(today.getDayOfYear() < birthdate.getDayOfYear()) {
            age -= 1;
        }
        return age;
    }

    @Override
    public String toString() {
        return String.format("%s(%d)", name, getAge());
    }


    public static void main(String[] args) {
        // 山田太郎(35) *2015/02/20時点
        System.out.println(new 日本人("山田太郎", "1980-01-01"));

        // 田村ゆかり(17歳と263ヶ月) *2015/02/20時点
        System.out.println(new 永遠の17歳("田村ゆかり", "1976-02-27"));

        // 堀江由衣(17歳と257ヶ月) *2015/02/20時点
        System.out.println(new 永遠の17歳("堀江由衣", "1976-09-20"));

        // 井上喜久子(17歳と400ヶ月) *2015/02/20時点
        System.out.println(new 永遠の17歳("井上喜久子", "1964-09-25"));
    }
}

class 永遠の17歳 extends 日本人 {
    private static final int SEVENTEEN = 17;

    public 永遠の17歳(String name, String birthdate) {
        super(name, birthdate);
    }

    private int howManyMonthsHasPassedFrom17(OffsetDateTime today) {
        int pastYears = today.getYear() - birthdate.getYear() - SEVENTEEN;
        int surplus = today.getMonthValue() - birthdate.getMonthValue();
        if(today.getDayOfMonth() < birthdate.getDayOfMonth()) {
            surplus -= 1;
        }
        return (pastYears * 12) + surplus;
    }

    @Override
    protected int getAge(OffsetDateTime today) {
        return SEVENTEEN;
    }

    @Override
    public String toString() {
        OffsetDateTime today = OffsetDateTime.now();
        return String.format("%s(%d歳と%dヶ月)", name, SEVENTEEN, howManyMonthsHasPassedFrom17(today));
   }
}