You can run other files inside PowerShell by doing
& "C:\PathToFile\File.ps1"
Your birthday example has the right idea but needs fixing. You can avoid most of the mistakes if you look for lessons online for PowerShell.
Here you are saving to $Readhost, but you never used it again in the script. Maybe you meant to use $birthday in this line instead. Also the Read-Host cmdlet has a parameter that displays a message which you can use to avoid using Write-Host in the previous
line.
$Readhost = Read-Host
This line you have the right idea of changing the string to a datetime, but it will only change $birthday temporarily because you aren't assigning (saving) your results by using =, and because there are no = it will send the reulst to your screen instead.
Then again you don't really need to assign in this line, in fact you can remove this line entirely and apply [datetime] to the next $birthday instead.
[datetime]$birthday
This line has two issues, you are trying to cast $now to a datetime even though it already is a datetime, and you are trying to subtract by $birthday which is still a string type (what you originally did in the previous line does not change birthday's type
permanently).
$age = [datetime]$now - $birthday
