It's very easy to gather this information for your records.
Check with the solicitor you used when buying the property, they may have kept a copy.
If you have a mortgage, your survey may say how old the building is.
Your local authority may have further details about when planning permission was granted.
Ask neighbours in the same sort of properties if they know the age of their house.
Check the property deeds for further information about when the house was constructed.
If you have lost the deeds to your property you should inquire with the land registry to get a new copy of them.
Hope that helps.