The so-called metal "Ages" happened at different times in different places. The Druids may have been spoken of by that name during the time of the Roman Empire, but that doesn't mean they "originated" in that time.
If anything, the late-coming Celtic-speaking people are earlier "practitioners of cultural appropriation".
"Bertrand and Reinach both maintain the pre-Celtic origin of Druidism."
--Wright, Druidism the Ancient Faith of Britain
"so we must pass on to the non-Celtic natives, who had another religion, namely, druidism, which may be surmised to have had its origin among them."--Rhys, Celtic Britain, p.69
"Professor Rhys goes so far as to refer Druidism to the Silurian race, because Caesar mentions Britain as the birthplace of that cultus, and it is of a character which he considers non-Aryan. It is almost certain that second-sight and other ecstatic moods must be referred to the pre-Celtic races."--MacBain, Celtic Mythology and Religion
"The monuments we call Druidical, must be appropriated, exclusively, to the Aborigines of the midland, and western divisions. They are found in such corners, and fastnesses, as have, in all ages, and countries, been the last retreat of the conquered, and the last that are occupied by the victorious."--Davies, Celtic Researches
"The Celtic invaders of Britain did not create Druidism, and, neolithic or not, it was at all events pre-Celtic."--Holmes, Caesar's Conquest
Therefore, who are the "non-Celtic natives" of Britain?
"Strabo reports of Posidonius ... 'The Cassiterides are ten in number, and lie near each other in the ocean, towards the north from the haven of the Artabri: one of them is desert, but the others are inhabited by men in black cloaks, clad in tunics reaching to the feet, and girt about the breast; walking with staves, and bearded like goats. They subsist by their cattle, leading for the most part a wandering life. And having metals of tin and lead, these and skins they barter with the merchants for earthenware and salt, and brazen vessels.' ...
Solinus ... states that 'a stormy channel separates the coast which the Damnonii occupy from the island Silura, whose inhabitants preserve the ancient manners, reject money, barter merchandise, value what they require by exchange rather than by price, worship the gods, and both men and women profess a knowledge of the future.'"
--Skene, Celtic Scotland,v1
"The wide extension of the Ligues westward is in agreement with the language of Eratosthenes. According to Strabo (2. i. 40, p. 92) this old geographer taught that there were three forelands projecting from the north—the Peloponnesian, the Italian, and the Ligurian—between the first and second of which lay the Adriatic, and between the second and third the Tyrrhenian Sea. When we remember the high reputation and the real merits of Eratosthenes, it is astonishing how little attention has been drawn to the fact that he calls the Spanish peninsula the Ligurian."
--Guest, Origines Celtae
"The swarthy complexion and curled hair of the Silures, together with their situation opposite to Spain, render it probable that a colony of the ancient Iberi possessed themselves of that territory."
--Tacitus, Agricola
These are the indigenous people of Western Europe who had the stone circles and nemetons, and the Druids. The Celtic language evolved from the Ligurian, and IMO, is proven by Whatmough and Sims-Williams.
