After the First Death triggers the immortality, the immortal now has full access to their healing abilities and will stop aging. In the original film, immortals are able to sense each other even before the First Death. In the live-action TV series, immortals have no access to their powers at all until after the First Death, including the Buzz which allows them to sense others of their kind. Other immortals can sense a pre-immortal. This makes them vulnerable to fully realized immortals who wish to steal their Quickening before they can realize their power and become trained fighters, just as the Kurgan tried to kill Connor MacLeod before the young Highlander knew his true nature.