Hi Martin,
you are selecting properties from a snapshot object and not from a virtual machine object. A snapshot object has different properties. You can use the following code:
Get-VM |Get-Snapshot |Select-Object-Property @{N="VMHost";E={$_.VM.VMHost.Name}}, @{N="VM";E={$_.VM.Name}}, Name, Description, SizeMB, @{N="DaysOld"; E={((Get-Date) -$_.Created).Days}} |Where-Object {$_.DaysOld -gt7} |Export-Csv"C:\Filename.csv"-NoTypeInformation-UseCulture