How old is that sea ice?

Sea-ice age is interpreted as a reconstructed history of survival and movement rather than a direct map of thickness or strength. The essay explains first-year and multiyear categories, parcel tracking, and the important product rule that retains only the oldest category within each grid cell. A hypothetical mixture demonstrates why identical colours need not imply identical fractions of older ice. Deformation, melt, snow, and travel history further limit age as a thickness proxy. The published decline in the oldest category is retained with its late-summer timing and comparison period, while the need for successive summer survival explains why a winter extent rebound cannot immediately rebuild old ice. The resulting reading method asks for product, week, categories, and compatible complementary measurements, not a promise about travel or habitat.