We did not hold back younger children from dating / getting married because an older sibling was still single.
Haimy – You asked “’Has Daas Torah weighed in on this difficult question”? Since as you state it is a minhag I am not sure Daas Torah has to give a blanket opinion. As with most questions relating to shiduchim individuals have to consult with their “rav” or whoever they ask their hashkafa shailos to.