Get-DaysLeft will tell you how many days you have left to live for various life expectancies. 100 is a nice round number. 82 is a good long life. 76 is the current life expectancy for the US. 65 is not uncommon.

Get-DaysLeft

$today = (Get-Date)
$bday = $(
  $DOB = Read-Host "What is your birthday?"
  if ($DOB){ $(get-date ($DOB)) }else{$(get-date)} #Default to today to avoid errors when nothing is entered.
)
write-host "You were born on $($bday.ToLongDateSTring())."
write-host "You have lived $(($today - $bday).Days) days."
100,82,76,65|%{ write-host "If you live to be $_ years old, then you have"(( $bday.AddYears($_) ) - $today).Days "days left to live."}
Write-Host "Make the most of them!"
Write-Host "But remember, there is only now."

# Credits:
# Lucy for asking how many days old she was on her birthday.
# Read-Host trick courtesy of Robert: http://social.technet.microsoft.com/Forums/windowsserver/en-US/6f3b8b60-78e1-4f4a-9499-e260a16509d8/readhost-default-value
# May this motivate you more than it depresses you. Remember only humans track time, everything else lives in the present.