File Age monitoring - By Type

Create a batchscript datasource using active discovery.

Make the appliesto:
pathtofiles

Use the wildvalue as the unique identifier. 

Use this as both the collection and discovery scripts:

$hostname = '##SYSTEM.HOSTNAME##'
$pathToFiles = '##pathtofiles##'

Invoke-Command -ComputerName $hostname -ScriptBlock { 
    $files = Get-ChildItem -Path $pathToFiles -File -Filter '*.bak'
    foreach ($file in $files){
        $wildvalue = $file.Name
        $age = [math]::Round((Get-Date).Subtract($file.CreationTime).TotalHours, 2)
        
        # For discovery
        Write-Host "$wildvalue##$wildvalue"

        # For collection
        Write-Host "$wildvalue.age: $age"
    }
}
Exit 0

Change the filter on line 5 if you want to look at other files.

Make your datapoint called age, use key-value pair, with the key being

##WILDVALUE##.age

Set a property on your device (or group of devices) called pathtofiles where the value is the path to the files you want to include.