How to check how old this scotch whisky is?
For blended whisky, the age is not generally considered a relevant factor. There will be a range of ages and whiskies in there, chosen to make the end result.
Unless a particular blend has ages specified (the Grant's Family Reserve doesn't), all you can guarantee is that the youngest whisky is 3 years old.
As an example, another Grant's whisky, the 12 year old, has whiskies that are 12 years old and older blended into it.