Opponents have branded the claims "shocking" and a "new low" for the SNP.
Mr Robertson made the claims in a column for the National newspaper, as he discussed the rise in support for independence since the last referendum in 2014.
"This could be in significant part because of the underlying change in the electorate with roughly 55,000 predominantly Yes supporting 16-year-olds joining the electorate and 55,000 predominantly No supporting older voters passing away every year.
"Since 2014, this had added around 333,000 voters to the electorate with a likely net gain of over 100,000 for independence."
But the claims came under fire from Tory leader Douglas Ross on Twitter.
He said: "Disgraceful and deeply disappointing comments from @AngusRobertson - suggesting that the most vulnerable age group, who have been hardest hit through the tragic loss of so many lives throughout the pandemic, are a boost to his independence obsession. A new low for the SNP."
Labour MSP James Kelly added: "This shocking from Angus Robertson. What sort of mindset does he have to write ‘55,000 predominantly No supporting older voters passing away every year."
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