I'd argue that the date could be that of the first memory the child has.
This is a date which will change over time, and be very difficult to
establish, particularly in the early years.
Most adults I know don't recall anything before their third birthday, so I
would argue that up at least to that point they were still part of their
mothers.
Alternatively, it could be the date the child becomes independent, which is
of course rather later.
Regards,
Darren