After the data table is sorted by the age column in ascending order, the cycle is conducted by the age column. When the age is equal to the age of the previous row, n=n+1, otherwise, the cycle is withdrawn.Thank you!

dt = Open( "$SAMPLE_DATA\Big class.jmp" );

r = N Row( dt );

n = 0;

For( i == 2, i <= r, i++,

If( dt[i - 1, "age"] = dt[i, "age"],

n = n + 1,

stop loop

);

);