For the late Middle Ages and early modern era, with the exception towards the end of the time frame of Scandinavia, life expectancy can best be expressed with two metrics. First, about half of people born died before age 16. Of those who survived, the majority would die by 50.

But it's important to keep in mind that "old" isn't just a relative term. That is, even if the "average" life expectancy was around 30, that doesn't mean biological definitions of child, young adult, adult, middle-aged, elderly compressed from 85 years into 30. It's true that medieval Europe didn't structure its society into age-sets like some regions (the Masai in Kenya and Tanzania are a famous example of that). But recognition of divisions by age wrought through human biological change, above all a tripartite division between childhood, adulthood (sometimes "youth"), and old age, is nearly everywhere you look.

In early medieval German law codes, the wergild for killing a person above 50 was much lower than for killing someone younger, and continued to decrease with age. Similarly, in late medieval slave markets, old enslaved people were bought and sold for significantly less than their younger fellow sufferers. In these cases, the amount of work/value was seen as higher for younger people.

With women and men, we see it in perceptions of "beauty." The beautiful male is youthful, beardless, smooth-faced. When Beauty and Ugliness are allegorized, they are depicted as a young woman and an old woman. (Here, Beauty and Ugliness ally to beat Chastity, in an illustration from the Romance of the Rose.) Or we could skip from outside to inside. Old people in medical and philosophical texts are repeatedly referred to as experiencing a second childhood, with the decline of mental faculties and physical prowess.

In Julian's case, she probably experienced her illness while still secular (living in the world, not yet an anchoress). In this case, it would make particular sense for her to be referring to "youth" as pre-menopause, that is, still fully able to have children.

"Older" in the Middle Ages might have been measured in much the same terms it is on reddit, but old--and thus, young--had fixed biological meanings as well as relative social ones.