Get-ChildItem -Path c:\windows |
  # clone the objects and keep the properties you want/add new properties (like "age...")
  Select-Object -Property LastWriteTime, 'Age(days)', Length, Name, PSIsContainer |
  # change the properties of the cloned object as you like
  ForEach-Object {
    # calculate the file/folder age in days
    $_.'Age(days)' = (New-Timespan -Start $_.LastWriteTime).Days

    # if it is a file, change size in bytes to size in MB
    if ($_.PSisContainer -eq $false)
    {
      $_.Length = ('{0:N1} MB' -f ($_.Length / 1MB))
    }

    # do not forget to return the adjusted object so the next one gets it
    $_
  } |
  # finally, select the properties you want in your report:
  Select-Object -Property LastWriteTime, 'Age(days)', Length, Name |
  # sort them as you like:
  Sort-Object -Property LastWriteTime -Descending |
  Out-GridView
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