Why the noise from cascode transistors are negligible at low frequencies?
They do not contribute the the noise because of the degeneration. Having emitter/source degeneration in effect introduces negative feedback, greatly improving the performance of the transistors. For a more quantitative discussion see the end of this presentation. 

You end up with the following equation for the noise:
$$\frac{i_D}{1 + g_m r_o} \approx 0$$
Where \$r_o\$ is the degeneration. For high frequencies \$C_{gs}\$ provides an alternative current path, reducing the degeneration.