YEAH, I agree with @txnelson , if you really wanna do this by using if, you can do like below
Names Default To Here( 1 );
dt = Open( "$sample_data/big class.jmp" );
total_rows = N Row( dt );
For( i = total_rows, i >= 1, i--,
If( :age[i] == 14,
dt << delete rows( i );
wait(0.5) // wait for a while to let you see the delete action
)
);