Some 150,000 applications were made on Tuesday, with over 51,000 received from people aged 25-34.
By contrast, 1,215 were received from over-75s.
It was the biggest total received for a single day since the 2016 referendum campaign.
However it is unlikely all of the applicants will be new voters as the figures include people checking to see if they have already registered as well as those ineligible.
During the 2015 election campaign, some five million applications to register to vote were made from January 1 and the deadline on April 20 - yet the number registered to vote increased by only 1.4 million.