Something

alias age {
  if ($1 == -a) {
    %birthday = $ctime($$?="When is your birthday (dd/mm/yyyy)" 00:00:00)
  }
  if ($1 == -d) {
    %bdaydefault = $$?="What is your birthday (Default) (dd/mm/yyyy)"
    %birthday = $ctime(%bdaydefault 00:00:00)
  }
  elseif (!$1) {
    if (%bdaydefault) {
      %birthday = $ctime(%bdaydefault 00:00:00)
    }
    elseif (!%bdaydefault) {
      %bdaydefault = $$?="What is your birthday (Default) (dd/mm/yyyy)"
      %birthday = $ctime(%bdaydefault 00:00:00)

    }
  }
  else {
    %birthday = $ctime($$1- 00:00:00)
  }
  %curyear = $asctime(yyyy)
  %nextbirthday = $asctime(%birthday,dd/mm) $+ / $+ %curyear
  if ($ctime(%nextbirthday) > $ctime) {
    %curage = $calc(%curyear - $asctime(%birthday,yyyy) - 1)
    %timeleft = $duration($calc($ctime(%nextbirthday 00:00:00) - $ctime))
    /say I am %curage years old. I turn $calc(%curage + 1) years old in %timeleft
  }
  else {
    %curage = $calc(%curyear - $asctime(%birthday,yyyy))
    %nextbirthday = $asctime(%birthday,dd/mm) $+ / $+ $calc(%curyear + 1)
    %timeleft = $duration($calc($ctime(%nextbirthday 00:00:00) - $ctime))
    /say I am %curage years old. I turn $calc(%curage + 1) in %timeleft
  }
}