What people don't seem to realize is they're all TRIPLETS, so they were born at the same time, but they WERE born in a chronilogical order...
Alvin was born first, five minutes later Simon was born, five minutes later Theodore was born.
Then for The Chipettes, Brittany was born first, ten minutes later Jeanette was born, then five minutes later Eleanor was born.
They're ALL eight-years-old, but they IS a just a SLIGHT age difference; Brittany's the oldest of all of them, so the chronilogical order is Brittany, Alvin, Simon, Jeanette, Theodore, Eleanor.
In fanfics, however, I always write them as thirteen, lol.