My oldest daughter has microcephaly. Microcephaly is a symptom of over 200 different conditions and has varying degrees of severity. It can also start before or right at birth, or develop within the first three years of a child's life. That was my daughter's case. She was born with a normal head circumference which slowed by 8 weeks old, and by 5 months old, she was microcephalic.
At 4 years old, her head circumference is 44.3cm, the equivilant of a 14 month old, and is -3.7 standard deviations below normal.
She does have seizures, hers is complex partial epilepsy, but there are many kids who do not have seizures at all and other kids who have hundreds of grand mal everyday, to the point of status epilepticus. There is also something called relative microcephaly, where a child's head circumference is just small and there is no developmental delay. These cases usually happen when both parents are also microcephalic.
Some children die before or shortly after birth while others live long lives. We have some babies in our support group who lived only a few days and other families whose children just celebrated their 23rd birthdays.
If you have any other questions, feel free to ask.