Doctor Strange may yet harness the power of agelessness in the Marvel Cinematic Universe. In the 2016 Doctor Strange movie, Benedict Cumberbatch's future Avenger meets with the Ancient One who, as her name implies, has been around far longer than the average human. Played by Tilda Swinton, the spiritual leader is said to have been alive for hundreds of years, defending Earth from cosmic destruction and inter-dimensional nasties. In Doctor Strange's big turning point, it is revealed that the Ancient One has defied the laws of aging by drawing power from the dangerous Dark Dimension. Since this method of anti-aging is both very risky and highly forbidden, Mordo immediately accuses the Ancient One of hypocrisy, sending yet another of her followers down a path of darkness.
When Stephen Strange takes over the position of Sorcerer Supreme, it's assumed that he chooses not to make the same mistake as his predecessor, and therefore the Strange seen in Avengers: Infinity War must be aging normally, like any other human would. In the Marvel comic books, however, one of Doctor Strange's main abilities is an extended lifespan, similar to that of the Ancient One, and fortunately, this is something that can still be added to MCU canon.
The Ancient One and Strange's agelessness in the comics comes not from the Dark Dimension, but from passing a test against Death itself. Covered in 1974's Doctor Strange #4, Strange travels through the Orb of Agamotto and is led to the horseman know as Death. After an initial struggle, the Sorcerer realizes he cannot defeat or outrun Death and accepts his mortality, at which time the Ancient One arrives to reveal that this battle was part of Strange's final test. The Doctor's reward is to live agelessly, and although he can still be killed by external factors, such as a battle wound or slipping while getting out of the shower, Strange would not die from natural causes.
This vital story is certainly something that could feature in a potential Doctor Strange 2. The character's agelessness in the comics gives Strange an otherworldly, time-weary quality that makes him unique among the other Avengers who are either human or naturally have longer lifespans. Such a trait would be a welcome addition to Benedict Cumberbatch's MCU future and would help his character develop both in terms of power and the weight of responsibility on Strange's shoulders. It would also make the live-action version of Strange more faithful to the source material, but in a way that doesn't cast the hero in the same morally gray light as the Ancient One.
It could also be argued that this plot was partially set up in the first Doctor Strange movie. During the Ancient One's final moments, the mystic master reveals to Strange that drawing from the Dark Dimension was an evil she deemed necessary in order to defend Earth from the Multiverse's various threats. Swinton's character goes on to say that she sees her own rule-bending attitude within Strange. This scene gives Strange a reason to seek out agelessness, other than fighting off those gray hairs. The position of Sorcerer Supreme isn't exactly a vacancy that can be advertised online, and those who are deemed suitable, such as the Ancient One and Stephen Strange, may need to extend their lives in order to ensure the dimensions are always guarded.
