If we look at sub-saharan Africa then the times for categorising these "ages" are a bit shifted and stretched compared with Asia, Europe and North-Africa.
Metal-using Africa Farming societies in Africa developed after the origins and spread of livestock pastoralism throughout the continent. Likewise, the early use of metallurgy by farming communities was not developed independently in Africa until around 3000 BCE. Pockets of iron usage appeared in subsequent millennia but metal did not supplant stone in the south of the continent until around 500 BCE, when both iron and copper spread southwards through the continent, reaching the Cape around 200 CE. Although some details regarding the Bantu expansion are still controversial amongst archaeologists, linguists, and historians, the widespread use of iron does seem to have played a major role in the spread of Bantu farming communities throughout sub-Saharan Africa. Contact and interaction between hunter/gatherer, pastoralist, and incoming farming communities remains an important topic of interest in African archaeology today.
One example that fits the question and might almost be described as still "living in that old way" are the San in South Africa. They didn't adapt to metallurgy and even rejected sedentary life:
The San, the first people in South Africa: The ability of Later Stone Age hunter-gatherers to sustain themselves was seriously challenged at least three times in the past 2 000 years. Firstly, this occurred with the southward migration of the Khoikhoi herders into the western half of the country. Although they appear to have developed a symbiotic relationship with the hunter-gatherers, they converted individuals to herding, and therefore weakened hunter-gatherer social cohesion.
Secondly, hunter-gatherers were challenged in the north and east of South Africa, as Iron Age farmers (Nguni and later Sotho nations) had settled in the summer rainfall regions within the last 1 800 years to grow crops and tend their stock. They also lived alongside hunter-gatherers, particularly in the Drakensberg region, and developed a working relationship with them. However, they became more and more powerful in terms of population size and land ownership. Finally, the death knell came with the arrival of European colonists whose commandos with guns and horses decimated the hunter-gatherers within two centuries. Some of this history is reflected in the rock art of the later Stone Age.
Also in Africa you have the well studied and famous Hadza people:
Since the 18th century, the Hadza have come into increasing contact with farming and herding people entering Hadzaland and its vicinity; the interactions often were hostile and caused population decline in the late 19th century. The first European contact and written accounts of the Hadza are from the late 19th century. Since then, there have been many attempts by successive colonial administrations, the independent Tanzanian government, and foreign missionaries to settle the Hadza, by introducing farming and Christianity. These efforts have largely failed, and many Hadza still pursue virtually the same way of life as their ancestors are described as having in early 20th-century accounts. In recent years, they have been under pressure from neighbouring groups encroaching on their land, and also have been affected by tourism and safari hunting.
In Brazil there are perhaps a number of peoples fitting the search criteria. Although most of these are simply not really known to the Western mind. One tribe still living as hunter-gatherers with known contact, and rejecting it, are the Pirahã, description has that they even reject using stones:
The Pirahã are supremely gifted in all the ways necessary to ensure their continued survival in the jungle: they know the usefulness and location of all important plants in their area; they understand the behavior of local animals and how to catch and avoid them; and they can walk into the jungle naked, with no tools or weapons, and walk out three days later with baskets of fruit, nuts, and small game.