Well, Katara is 85 years old in The Legend of Korra, and Korra herself is 17 which would have put Katara at 68 years old when Korra was born. It is unknown if Toph, and Sokka where living during those first 16 years. As for Aang we already know he had to pass on before Korra's arrival due to the nature of the Avatar cycle. So, I'd assume barring they where alive during a period of that 16 years of Korra's life we didn't see that they where between 65 - 73 when they passed. While not necessarily fitting the current real-world life expectancy, it's still a rather ripe age considering all they went through.
P.S - Still make's me sad to know they are gone in LOK