This depends on the whanau.
For example the whanau of the writer are fortunate to live long lives so our kaumatua are in their 80´s, 90´s and 100´s. In our whanau it is quite common to have 4 generations alive at the same time, and in a few cases, 5 generations. Three generations alive at one time is the norm.
My generation ranges in age from 60´s down to 20´s. There is no way that any of my generation would consider being kaumatua as we have at least another 20 years to go!
However, other whanau live shorter lives and therefore the age of their kaumatua reflect this.
Some people today think that when they turn 60, get grey hair and a tokotoko (walking stick) that they automatically become a kaumatua. However, the people decide, not the person themselves.