The Equality and Human Rights Commission in Wales accused the UK government of a "communication failure" to let women know their pension age would increase in line with men at 65.
In 2011, the government brought the deadline forward from 2020 to 2018.
It said letters were sent to the women advising the changes.
The age for both men and women will increase further to 66 at the end of the decade.
Kate Bennett, Wales' EHRC's national director for Wales, told the BBC's Sunday Politics Wales programme: "What has been a serious problem is the failure to communicate properly with women to let them know the situation.
"I'm aware of the fact that women who've planned their retirement for a very long time have suddenly discovered that actually they're not going to get their pension as quickly as they thought and have had to carry on working longer.
"This really is a failure on the part of the government and, although we support the change, it should have been implemented much better."
The UK government said it had notified the women affected and would not be revisiting the state pension age arrangements.
"The cost of prolonging this inequality would be several billions of pounds," it said.
Elizabeth Conway, from Tonteg, Rhondda Cynon Taff, retired as a nurse at 58, expecting to start receiving her state pension when she turns 63 this year.
"You think that you're going to get your pension at a set age which you could then add to another pension if you've got one and it would give you a reasonably good life but now I have to work and it's quite hard," she said.
The way changes to the state pension age were made has been criticised, with thousands of women claiming they were given "little or no notice".