With some fake data, this works:
% fake data
f = -pi:0.01:pi;
allspectrums_cortex_young = sin(2.*f + pi/3) + 0.05*rand(size(f));
allspectrums_ventricle_young = sin(3.*f + pi/2) + 0.05*rand(size(f));
allspectrums_cortex_elderly = sin(2.5.*f + 0) + 0.05*rand(size(f));
allspectrums_ventricle_elderly = sin(0.75.*f + pi/6) + 0.05*rand(size(f));
% code
figure;
l1 = plot(f, allspectrums_cortex_young, 'r','DisplayName','ventricle cortex');
hold on
l2 = plot(f, allspectrums_ventricle_young, 'color', [0.8 0.7 0],'DisplayName','ventricle young');
l3 = plot(f, allspectrums_cortex_elderly, 'b','DisplayName','ventricle elderly');
l4 = plot(f, allspectrums_ventricle_elderly, 'color', [0 0.75 0],'DisplayName','cortex elderly');
legend('location','best');
hold off
xlabel('Frequency (in hertz)');
ylabel('Power');
title('Young vs. Elderly Power - 4th Ventricle and Cortex')
