It's a milestone of growth of the baby. It may vary, have a age period range. but if its late more than the normal range need to be evaluated. Look for significant birth history as not cried immediately after birth or any consanguinity in parents or family history showing genetic disorder. Hence Need a check up ..

Get ur child checked by a pediatrician....clinical examination is necessary....but by 6 months babies shd do it.  

Holding neck, turning from back to front and front to vack , sitting etc all these are milestones of development which vary in different children There is no absolute age for a milestone to be achieved. Usually there is range of few days to months and a cutt off upper limit beyond which is a cause of concern. If u have any concerns regarding developmental milestones better co consult a pediatrician for assessment.