It's a common fantasy trope to say that a character is "X winters old". I was wondering if this was an invention from literature or something people actually did. Does this fact have a name so I can look it up and compare across cultures and time periods? (rural 15th century in the Andes or urban Roman Antiquity for example)
Previous answer here indicates it may have been a thing in Early Medieval Viking culture to track winters for age, seemingly until they were considered adult enough; https://www.reddit.com/r/AskHistorians/comments/1jp7pa/how_did_people_keep_track_of_how_old_they_were/
And previous answer here fits my instinct that is more associated with older than newer times and that it was (probably) more common among the rural lower classes while upper-classes would (probably) be the first to regularly use calendar years and calendar-based age.
I can say that the in the ninth century Carolingian monk Hildemar of Civate describes life stages and the allocation of diet according to conventional calendar years, as described by Lynda L. Coon's "Dark Age Bodies: Gender and Monastic Practice in the Early Medieval West" (2011). And in the fourteenth century French manuscript Les Douse moys Figurez age is described in terms of normal years according to Elizabeth Sears' "The Ages of Man: Medieval Interpretations of the Life Cycle" (1986). Nevertheless, Chris Gilleard in "Aging and Old Age in Medieval Society and the Transition of Modernity" (2002) notes that the classical life-stages these sort of writings usually deal with were not strictly defined by age although writers like I have mentioned occasionally defined their own specific borders. And this changed over time, he describes the more formal, mechanical and material culture that developed with the increasing use of mechanical time-keeping that promoted recording age more regularly. And this influenced the perception of age as more precise.
It's worth noting that keeping age in years according to a calendar system is also easier than seasons, given that seasonal conditions vary in how quickly they change, how severe they are, and whether they go in and out which is why we usually measure seasons by calendar because the actual conditions vary. It also means depending on what seasons you were born in you might sound older than you really are or be grouped in age with someone younger. Birthdays track the precise age in a more efficient way, and are also a popular social institution so it's natural to incorporate them into time-keeping.