Try using unique on just the ID's part of the array (I'll assume you're input is called "array"):
[ids, ii, jj] = unique(array(:, 1));
This will get you the first column of output. ii stores the index of (I believe) the first occurrence of each corresponding value in ids in array(:, 1). (i.e. ids(1) = array(ii(1),:)). Thus, you can get the ages using:
ages = array(ii, 2);
To get the last column, the sum of the ages, you'll use the third output of unique, jj. For each id in the original array, jj gives its index in the unique ids array. This will give you the sum of the ages:
sums = accumarray(jj, array(:, 3));
There a lot of different ways to use accumarray. When you give it two column vectors (as above) it essentially does this:
for i = 1 : numel(jj)
sums(jj(i)) = sums(jj(i)) + array(i, 3);
end
