Why "little" louis and "baby" ping?

Once upon a time, there were 2 little good friends...

little louis: halo, green sour apple... :-p
baby ping: little boy, u are very naughty... u know?
little louis: hey, I am not a boy, we r of same age...
baby ping: but... but... I am 9 days older than u, call me "big sister big"!
little louis: no way... it's just 9 days! In fact, my mom told me that I stayed longer in her womb, so it might happens that I am older than u...
baby ping: so what? If u dun like "boy", fine... I just call u "little louis", okie?
little louis: hmm... okie, but I will have to call u "baby ping" in that case...
baby ping: y?
little louis: hahaha... coz the level of "baby" is still lower than "little"... I am still older than u! :-p
baby ping & little louis: LOL~