The above image is a poster dating back to June 1837 which outlines the new law at that time regarding the registration of all births and deaths in England and Wales. All births and deaths now had to be registered with a local registrar within 6 weeks of the event, after which a cost was incurred.
Prior to June 1837 records of baptisms, marriages and funerals were kept by local churches (as they still are today) but these were different from birth and death registration and can be misleading for the genealogist. For instance, in my own family research I have found instances of 2 or 3 children of different ages being baptised at the same time. The reason posssibly being that the family may have moved away from their home town or village, perhaps into a larger town or city, but wish to have their children baptised back at their local parish church. They therefore waited until there were a few children for baptism to make the journey worthwhile.
The problem this presents for the family history researcher is when a baptism is used to estimate the year of birth it could actually be 3 or more years out. It is therefore worth studying the original document to see if the age of the baby or child is provided.