The decision has some wondering what Trudeau will do for young Canadians when seniors enjoy better financial health than they ever have — some of whose pocketbooks will only grow healthier with the upcoming budget.

It wasn't such sweet news for millennial taxpayers who now — again — have to pay for a retirement program whose costs are expected to more than double over the next 15 years. And then keep growing.

Justin Trudeau had good news for seniors when he said he'd roll back eligibility for Old Age Security (OAS) from 67 to 65 years old in next week's budget.

Justin Trudeau announced that he would lower the age of eligibility for OAS payments to 65 from 67 years old in an interview with Bloomberg on March 17, 2016. (Photo: Bloomberg via Getty Images)

In 2012, then-prime minister Stephen Harper hiked the OAS eligibility age to 67, phasing in the change starting in 2023.

The decision would have eliminated an estimated $11 billion in annual spending up to 2030, which would have grown to $16 billion per year by 2050, according to actuarial reports.

But Trudeau, who promised the reversal during last year's campaign, called Harper's move a "mistake," and said he was changing it "because how we care for our most vulnerable in society is really important."

He didn't outline precisely why he's changing it back, but the C.D. Howe Institute's Alexandre Laurin said he did it to protect low-income seniors who would have had to rely on social security while waiting two more years to retire.

The government could pay for this any number of ways, such as budget cuts and "prolonged low interest rates," Laurin said.

He also said it could "probably be financed by a small increase in the GST rate, or in income tax rates."

"How we care for our most vulnerable in society is really important."

Trudeau may feel he's protecting the vulnerable, but BMO study from 2014 shows the average Canadian senior is anything but financially frail.

Seniors are "four times richer than their parents were at the same age in the mid-1980s," the report said. More of them have jobs, own homes and enjoy higher returns from their investments.

"Today, the typical senior is nearly nine times richer than the typical millennial, a wealth gap between similar age groups that has more than doubled since 1984," it said.

And despite that, millennials are paying for a retirement system that benefits people who are richer than they are.

What is 'OAS'?

OAS is Canada's biggest pension program. It doles out monthly payments to seniors aged 65 years or older, with amounts based on how much money they make per year.

Unlike the Canada Pension Plan (CPP), it is funded out of general revenue, meaning every taxpayer pays for it.

A senior who makes up to $73,756 receives a maximum monthly payment of $570.52. A recovery tax, or clawback, kicks in for any amount above that, up to $119,398, the highest income at which you can receive OAS.

As the Fraser Institute noted, a couple that makes a combined income of $147,512 could still be pulling in full OAS benefits.

That means any millennial man making $42,160 — the average salary for a male aged 25 to 34, according to Maclean's — is topping up the income of a household making over three times as much as he does.