Here on earth, the year is measured by how long it takes our planet to make one trip around the sun. On Roshar, the year is measured by the Highstorm cycle; 1000 days of 10 10-week months. This would imply that the characters in the Stormlight Archive are older than we might assume given the number of years they've lived. Let's take Shallan for example: In WoR, she says she's 17 years old, meaning she's been alive for roughly 17,000 days. If we were to convert that into earth time, that would mean that Shallan is about 46 earth-years old.