How can I use PowerShell to enable NTFS compression, in place, on log files older than x days?

The easiest solution, as PowerShell support for file operations is still rather lacking, is to create a PowerShell script to call the compact.exe utility and set it up as a scheduled task. Because of the space in the path name, you want to call compact.exe directly, instead of using Invoke-WMIMethod and the CIM_DataFile class (which will cause a lot of extra effort to deal with the space in the path).

Assuming an age of 3 days for X, your PowerShell script would look something like:

$logfolder="[location of the first logging subfolder]"
$age=(get-date).AddDays(-3)

Get-ChildItem $logfolder | where-object {$_.LastWriteTime -le $age -AND $_.Attributes -notlike "*Compressed*"} | 
ForEach-Object {
compact /C $_.FullName
}

$logfolder="[location of the next logging subfolder]"

Get-ChildItem $logfolder | where-object {$_.LastWriteTime -le $age -AND $_.Attributes -notlike "*Compressed*"} | 
ForEach-Object {
compact /C $_.FullName
}

...

The second condition there is to speed up the script execution by skipping over already compressed files (which would be present after the first time this script was run). If you wanted to, or had a lot of different logging subfolders, it would probably make sense to make a function out of that repeated PowerShell code, which would be a fairly trivial exercise.

The repeated code can be avoided by using an array and a foreach loop:

$logfolders=("D:\Folder\One","D:\Folder\Two")
$age=(get-date).AddDays(-3)

foreach ($logfolder in $logfolders) {
    Get-ChildItem $logfolder | where-object {$_.LastWriteTime -le $age -AND $_.Attributes -notlike "*Compressed*"} | 
    ForEach-Object {
    compact /C $_.FullName
    }
}

.....