The drop in under-18s is not unexpected as the $1000 one-off payment was the only incentive for children to join the savings scheme.
But it was thought the change would have had only a minor impact on adult sign-ups.
For employed adults the $1000 is a small incentive compared to the annual benefit they can receive of up to $521 from government (if they put in at least $1043 of their own money) and the 3 per cent contribution employers are required to put in.
Claire Matthews, a KiwiSaver expert at Massey University, said part of the drop in sign-ups would be attributable to fewer children being enrolled, but that could not account for all of the fall-off.
Matthews said the trend was disappointing but not a surprise given human behaviour.
"To me what that signals clearly is a lack of financial literacy because within two years you have $1000 from the government in terms of the member tax credit.
"Over the life of KiwiSaver, $1000 is insignificant."
Matthews said people would have made a decision based around not getting something right now.
"People focus on the present rather than the future and the fact that money is something they are not getting now.
"It is really disappointing and concerning."
Matthews said the numbers showed the government should have undertaken a public education campaign which explained KiwiSaver members could still get up to $20,000 from the government over a 40-year working lifetime if they contributed $1043 a year themselves.
Matthews said despite the change KiwiSaver remained attractive to Kiwis wanting to save for retirement.
David Boyle, head of investor education at the Commission for Financial Capability said KiwiSaver was designed to get adults in the workforce saving and any drop in the sign-up rate from that group would be concerning.
Boyle said enrolments would naturally slow down over time as the scheme reached saturation point, which would likely happen in the next five to 10 years.
He said around 3.8 million Kiwis were eligible to join KiwiSaver, including children, but unless it was made compulsory there would always be some people who either did not get around to signing up or did not want to because they were saving through other means.
There were also those who could not afford it, he said.
The Government ruled out auto-enrolling all over-18-year-old workers last year because of the cost.
Boyle said it would be disappointing if the total number of people in KiwiSaver didn't get close to three million.
As of January there were 2.59 million people signed up.