Doesn't work with [23,12,56,2,80], should be [56,80]

not so good.
Because your solution are not readable
and this is the main thing

Not so good.
Because sort giving O(n) log n complexity of the algorithm.
You can solve it with linear complexity.
For exemple:

function twoOldestAges(ages){
var max = 0;
var secMax = 0;

for (i = 0; i<ages.length ; i++){
    if(ages[i] > max){
        max = ages[i];
    }else if(ages[i] > secMax){
        secMax = ages[i];
    }
}
return [secMax, max];

}

It's the optimal way of solving this kata.