The age of Arctic sea ice can be determined using satellite observations and drifting buoy records to track the movement of ice floes, and can be a rough indicator for ice thickness.
This NOAA PMEL animation shows the age of the ice at the end of each winter since 1987. Paler colors indicate older ice, with light blue indicating 4-year ice to white indicating very old ice.
In the 1980’s, 26% of the Arctic winter ice pack consisted of thick ice built up over multiple years. After 2007, older ice diminishes rapidly and is replaced by younger ice. Starting in 2011, very old ice remains only along the Canadian coast.