Humans in the world of Bleach are born, live and die in the same stretch of time as those in the real world. Unlike the Soul Reapers, who have a body made up of Reshi and live for thousands of years, humans have a physical body within the world. Even those with supernatural powers, such as Fullbringers and Quincy, are chained to the natural order of entropy. This has been shown through the aging of characters like Yasutora Sado, Uryū Ishida, Masaki Kurosaki and Ryūken Ishida.
However, there is a strange phenomenon when it comes to the Sternritter under the service of Yhwach. Its members range in age from their appearance, and many -- such as Jugram Haschwalth and Bazz-B -- have been hidden within the Schatten Bereich for hundreds of years. Although they are powerful Quincy, they are still all human, and as such should follow the laws of nature in the same manner as Uryū, Masaki and Ryūken. Yet these Sternritter seemingly age more like the Soul Reapers, keeping young adult appearances over a span of hundreds of years. Some look older than others, and the entire aging system appears inconsistent. There are two likely reasons why this oddity occurs.
Yhwach Influences the Sternritter's Spirits With His Shrift
The Sternritter were not all recruited at the same time. Whilst Jugram was recruited as a child in a medieval-like period, Äs Nödt seems to have been brought into the Wandenreich during a time of modern medicine due to Bleach's manga panels showing him attached to life support machines. This implies that aging within the Sternritter has not been halted but is greatly reduced, similar to that of the Soul Reapers. It is more than likely influenced by the insertion of Yhwach’s Shrift -- the enhancements of the Sternritter’s power.
As it has been confirmed that Yhwach ages much slower -- likely due to his power ‘The Almighty’ and its effects of dispersing his soul, making his soul body far superior to his physical one -- it makes sense that imprinting his soul into his followers gives them a prolonged life much like his own. A part of Yhwach is engraved into the Sternritter, granting them a letter and power, but ultimately it is still his soul within them. That would explain why Quincy such as Uryū, Masaki and Ryūken aged at a normal human rate.
The Schatten Bereich’s Effects on the Sternritter
Another factor driving the Sternritter's long lives in Bleach is where they have been hiding for many years. The Schatten Bereich is the shadow to the Soul Society, placed within the Spirit Realm and hidden from those who are unwelcome to it. The benefit of a human remaining in the Spirit Realm for hundreds of years is that it seems to replace their physical body with their spiritual body, making them more similar to a Soul Reaper than they would likely care to admit. The Quincy follow the same laws of nature as the Soul Reapers within the Spirit Realm, whereas they would have passed on long ago had they remained in the human world.
Whilst there is no direct evidence that either of these factors contribute to the Quincy's longer life span, the evidence that presents itself makes the most logical sense. Yhwach’s soul is so powerful that it remains beyond a human's life span. By enhancing those who serve him with that soul, they are granted a taste of ‘The Almighty’; this allows them to prepare for Blood War for hundreds of years, which suits Yhwach’s agenda. Considering they have lived in the Spirit Realm for so long and haven't aged since, the Schatten Bereich must have acted as a cryogenic chamber until the Wandenreich was ready to invade the Soul Society.