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All the Century Babies were born "with the century" - on the stroke of midnight on New Year's Eve of 1900. As they grew each displayed certain superhuman abilities and skills, as well as immortality (not aging beyond their 20's, late 30's or early 40's).
It has been theorised that they act as an immune system for the planet, their very existence protecting Earth. Each of them supposedly represents an aspect of the century into which they were born.
They are exist outside the system, neither alive or dead. They are not human, They are not part of the natural order as they are constructs created to do certain jobs.
While Century Babies are known to be immortal, they are not invincible, and can be, and often are killed, invariably as a result of the extraordinary lives they lead. Though they die their spirits live on.
Interestingly, after her death on the stroke of midnight December 31st, 1999 (the end of the 20th century according to consensus reality), Jenny Sparks was actually reincarnated as the "Spirit of the 21st Century", but it remains to be seen if she was the exception rather than the rule. Indeed, other Century Babies have continued living into the 21st century, including Elijah Snow and Dr Axel Brass.
The children of the Century Babies also apparently develop superhuman powers as a result of their lineage, though it is unknown if the powers are specifically determined by that of their parents' abilities. Known children of Century Babies include Jakita Wagner (daughter of Lord Blackstock) and Anna Hark (daughter of Hark).
Jenny Quarx visits a city/dimension where the spirits of previous "Jenny" Century Babies are living. According to an older Jenny Quantum, this city was created by Jenny Quantum's own powers after visiting the Doctor's garden, which houses the spirits of all the previous Doctors. It turns out that not all of Jenny's predecessors lived for an exact century, rather they lived for a certain period of time that they represented. Only relatively recently have the Jenny Century Babies been living for an exact century. Jenny's predecessors included Jenny Steam (the spirit of the 19th century), Jenny Revolution, Jenny Crusade, Jenny Inquisition, Jenny Plague, Jenny Ra, Jenny Fire and Jenny Stone.
The concept of the "century baby" is fatally flawed, as it grants a preference to the Gregorian calendar, and to the definition of "century" as starting in 1900 rather than 1901. (Addie Vochs of The Monarchy was a Century Baby born in 1901, but on the stroke of midnight on April 1st, the symbolical New Year according to certain horoscopes). However, according to a hypothesis put forth by The Doctor, the collective consciousness of humanity and their belief that the century starts during the xx00 year, not the xx01 year is what wills the Century Babies into being. When this issue was brought up to Jenny Sparks, she also blamed this on the fact that most of the world believes in the flawed definition and thus, she must conform to it.