Java, threadsafe class

import net.jcip.annotations.*;

@ThreadSafe
public class Person3 
{
    private final String firstName;
    private final String surname;
    private int age;

    public Person3(String firstName, String surname, int age) 
   {
        this.firstName = firstName;
        this.surname = surname;
        this.age = age;
    }

    public synchronized void birthday() 
    {
        age = age + 1;
    }
}