public long getAge(K key)
    {
        CacheableObject<V> co = cache.get(key);
        return co == null ? 0 : co.createdAt;
    }
public long getAge ( K key ) { CacheableObject < V > co = cache . get ( key ) ; return co == null ? 0 : co . createdAt ; }
@return System.nanoTime() when key was put into the map.