Parents tend to keep a lot of information about their children in their early to school years; a lot of this could be very useful in determining the birth date of a child. Certainly when our child was born not only did we buy a baby book to note down important moments in our babies first few years we given a few a well! A modern baby book allows the parents to keep information before a baby’s birth including scans as well as leaving lots of room to put in information about the birth. It is likely as well that there will have been plenty of correspondence between family members around the time of the birth and some of these letters could well have been kept as keepsakes. In fact letters between family members could contain plenty of opportunity to find out the birth date of a child. Birthday cards, baptism cards, first communion, confirmation and other religious events could all give clues as to the age of the child and it is likely that some of these could be in family records. It is quite possible that a family member keeps a diary and the birth of a child or family member is more than likely to be in it.
The Education Act of 1870 made it compulsory for all schools to keep records of their students. It is unlikely that a family will have the sorts of records that schools keep in their possession but they may have things like school reports and certificates. At my parents house they have all the reports of my brother, sister and I which can be a somewhat embarrassing read. Also in with them are the school reports of my mother as well. These can be used to narrow down the birth date of a child and to cross reference with other pieces of information. You may be lucky enough to find a class photograph or school photograph with the year on it, this could be very useful.
Some events are so important that the family gets them printed in the local newspaper. National newspapers became commonplace from the middle of the 18th century onwards but local newspapers were a lot later around the middle of the 19th century. If an event was important enough to be put into the paper it would not be unusual for it to be cut out and kept in the family records. Many of these newspaper cuttings could give an indication or even an exact date for the birth of an ancestor. Certainly newspaper cuttings to do with the birth will give the exact date but news about marriages or deaths/burial could give an age and therefore an indication of when the person was born. You may be lucky enough to find an obituary on an ancestor which could give you far more information than you anticipated. Not all ancestors were necessarily lawful god fearing people and it could be that you will find cuttings related to a crime that they committed. These usually detail the age of the criminal as well as their crime and sentence.