Thank you for posting on Microsoft Q&A.
You can use the DateTime.ParseExact method.
https://learn.microsoft.com/en-us/dotnet/api/system.datetime.parseexact
$dateString = '25-09-1973'
$date = [datetime]::ParseExact($dateString,'dd-MM-yyyy',[CultureInfo]"en-US")
$age = [datetime]::Now - $date
Best Regards,
Ian Xue
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