What do you mean by Neolithic age?

Neolithic simply means 'new stone age' which is the latter part of the broadly termed 'stone age', that is, the period when stone tools and weapons were used by man. This lasted from between approx. 3500BC to 2500BC and is characterised by the end of itinerant hunter gather societies and a shift towards farming and static, long lived communities.

From around 2500BC bronze was starting to be smelted and cast into weapons and tools and there was a cultural shift as this new technology was spread through trade and with it new ideas on rituals - barrows or burial mounds went from being elongated chambered long barrows that were reused again and again for hundreds of years to round shapes that were created for one primary burial with others only added ad hoc at later dates in the walls of the mound.

The major stone monuments were created in the Bronze Age, Stonehenge (which isn't actually a henge monument as its bank and ditch are the wrong way round) in its recognisable form and Avebury in Wiltshire, UK. Larger monuments demonstrate larger communities with the manpower for farming and community subsistence whilst others worked on purely ritual projects. Earlier communities could generally only manage a few upright stones for the entrance of a barrow that would be used over a long period without more investment in manhours due to lack of cohesion and manpower in one place.

The final era of pre-history (pre-history referring simply to an era before recognisable writing and recording of events; eras where writing and recording failed, such as the post Roman 'Dark Age' period in Europe is referred to as proto-history) is the Iron age, which lasted from around 800BC to the advent of the Romans in Northern Europe. Iron replaced bronze as it was far stronger and harder and made better tools and weapons. There was also another cultural shift towards the very end of the Iron age, away from flat, treeless plains and stone, such as Stonehenge, and towards forested, watery places, peat bogs, marshlands and springs for ritual practices.

Once the Romans took hold in Europe the pre-historic age ended.