I think that is a fun idea! Maybe we could draw pictures or fanart of them with cake or something, or just talk about them on their special day. We could also try to check the forum on their birthdays.
Their birthdays are in 2002 and the book was published 2003, so he must have used the real days he created them as their birthdays, and the book was set in the future.
If we use their 2002 birthdays, that means they are 12-13 years old. Wow, I did not realize Alien 9 was that old.
But, if they started sixth grade in 2003, they would be ages 12-13 at that grade, so they would be 24-26 now.
Also, Yuri is actually the oldest, but only by a couple months. Kumi is the youngest, her birthday is last, but she is more mature.
Kasumi's birthday is coming next July 1st.