Druidry has gone through four time periods, described by the Order of Bards, Ovates and Druids as I will lay out below. I will use BCE to indicate Before the Common Era,and CE to indicate the Common Era. The Common Era is the era that began with Jesus Christ.
PROTO-DRUIDRY (25,000 BCE to 600 BCE). Since much of this time period was pre-historical, it is hard to know much about Proto-Druidry, except that ancient people developed an understanding of deity and a reverence for nature. During this time period, the great stone monuments, such as Stonehenge, were constructed, probably between 4200 BCE and 1400 BCE.
CLASSICAL DRUIDRY (600 BCE to 600 CE). Druidry flowered during this time. A well-developed body of teachings and practices came into being, with schools where one could undertake a nineteen-year training to become a Druid.
UNDERGROUND DRUIDRY (600 CE to 1600 CE). With the arrival of Christianity, Druidry went underground. Druid teaching survived in folklore; in Bardic schools that continued to function, often secretly, in Ireland and Scotland; in tales and practices adopted and adapted by the Christian Church; in manuscripts of ancient tales and sayings recorded by Christian scribes; and in physical monuments, such as sacred wells and stone circles.
REVIVAL DRUIDRY (1600 CE to Present). The Renaissance got Europeans interested in ancient teachings, which were spread by the printing press. The Romantic movement sparked an enthusiasm for folk culture, and in Britain, people became interested in their Celtic ancestors. This led to a revival of interest in Druidry and the formation of Druid groups. Today there are well-established and recognized Druid orders in Britain and elsewhere in the world, practicing and teaching Druidry.
Some people question the authenticity of today's Druids, wondering how today's Druids can call themselves Druids when we lack solid, factual, written information on ancient Druid teachings and practices. This comes from a linear-time mindset in which, as we move further and further into the future, we move further and further away from Classical Druidry, which recedes further and further into the past and becomes lost in the mists of ancient time.
There is, however, another way to look at this. We might see time as circular, with God in the center of the circle. As we move closer and closer toward God, we come closer and closer to the spirit of Druidry. Authentic Druidry depends on closeness to God, not on proximity to a particular time period. We might note, for instance, that indigenous religions from around the world have an amazing similarity of teachings and practices - honoring of animals, reverence for the four directions, use of sacred music and dance, to name a few. The people practicing these indigenous religions are widely separated on the globe, yet the practices are very similar. This can be attributed to closeness - not to each other - but to God.
Time does not separate us from distant or ancient spiritual people. They and we are united in God.
It is also worth noting that any religion will evolve over time, especially if one is attuned to God. The Spirit of God is creative and active, not stagnant, and understandings of deity and religious practices that developed during one time period may not fit another time period. Therefore, even if we knew exactly what ancient Druids taught and practiced, it would make little sense for us today to adopt these ancient teachings and practices wholesale. We might think, for example, of the ancient Hebrew practice of animal sacrifice. This worked for the ancient Hebrews but is hardly appropriate for Jewish people today.