A spokesman for Glasgow City Council said not to do so would be unfair to other interest groups who are already charged for day services. However, the move has led to concerns that older people will give up using day centres, rather than pay.
Although the council says those on low incomes will be able to avoid the charges, support groups say many people who use the centre do not feel comfortable sharing information about their finances and will still surrender their day centre place.
Sheena Glass, director of Glasgow Old People's Welfare Association, said: "They have to complete a form to see if they are liable. We believe quite a few people will not have to pay, but through pride, some may give up their place."
The £15 a day charge could drive others into debt, she said.
Although three centres run by the association are to be subject to charges, the money will be paid to the council because it already funds the charity to run them.
Ms Glass said: "There are an awful lot of people who use our centres on benefits. But I can't say to my managers 'Help this person as much as possible', because it is not our debt.
"Everybody has forgotten these are human beings. The result is going to be that some stay in their house longer, and are less active. They will end up in expensive acute hospital beds, blocking people who really need them. The NHS will feel the brunt of this six months down the line."
Opposition councillors and charity groups have also complained about the way the decision was announced. A letter went out to those using the centres, arriving on the day the policy was discussed at a meeting of the city council's Health and Social Care Policy Development Committee.
Councillor Susan Aitken, the SNP's spokeswoman on health and social care, said a paper on the charges had been a travesty. "It was tabled in the knowledge officers had already issued letters telling people they could be hit with charges of up to £15 daily for day care services," she said. "There was no opportunity for scrutiny of the proposals and no scope to change what was planned."
Ms Glass said Glasgow Old People's Welfare Association had never been formally told elderly people who use the care centres were to be charged, but had learned on the day of the meeting from a council contact.
"Had we been given a bit more information we would have been able to help these people. No one really knows anything about it, and we didn't until people started bringing in their letters," she said.
A spokesman for Age Scotland said it was unclear what the council's rationale was for bringing in charges for a previously free service. "Services need to be sustained and it is vital older people can continue to rely on them without worrying about affordability," he said. "These charges would prove a false economy for the council should older people ration or even stop using services on grounds of cost. Recent research has identified loneliness as twice the health risk of obesity, so risking further isolation of older people will make life miserable for many of them and have serious implications for public health budgets."
A council spokesman said the charges were necessary because other users were already charged for care services.
The spokesman said: "The effect of new social care legislation is that we have to charge for day care services. Asking people to make a contribution to the cost of their day care services is about being consistent and fair to others who use social work services.
"If other care groups are asked to contribute financially for certain care services, it cannot be fair that one group is treated any differently." He said the council might face legal action for discrimination if all those who used social work services were not charged for the care they received.
But he added: "Those who cannot afford to pay will not be expected to pay towards the service they receive.
"Many other local authorities already charge for day care service and a significant majority of the cost of the Glasgow service will continue to be picked up by the council.
"All those who use the care centres have been sent letters and asked to complete a financial assessment form so that we can directly link the amount someone contributes to their ability to pay."
In response to concerns that some might withdraw from services rather than face means testing, the spokesman said: "We would take that seriously."
He added: "If people are intent on giving up the service social work would have a responsibility to the welfare of those individuals."
Giving up a service would be likely to lead to an assessment to see whether there was a "social care imperative" that they continue to receive it, the spokesman added.