Relating class to real life

A few weeks ago we discussed the different stages and ages of children when they typically reach certain milestones.  My brother is almost 8 months old and hasn't started crawling as a typical 8 month old would.  I have a cousin who is a week younger than him and she is very much ahead of him in all aspects and it is something my dad and step mom have been talking about for a while now, and are even considering taking him to get it checked out to make sure everything is alright with him. We are all praying it is just a minor setback and that nothing serious is wrong!