How old do girls have to be before they can have a baby?

Ovulation, a necessary precondition, usually starts at around age 12.5.

However, there have been anomalous cases where girls began ovulating and menstruating at a much younger age, including one girl who began menstruating at 8 months and became pregnant before her 5th birthday... she actually delivered (via Caesarian section) at the age of 5 years, 7 months, 21 days.