We were visiting the 7th century 'Chapel of St Peter's on the Wall' in Bradwell, Essex. My DS was about 6 and I tried hard to impress on him just how very ancient the chapel was. I got him to place his hands on the stone walls, saying "Just think, you're touching the same stone that people over 1000 years ago touched". And "People stood right where you're standing now, little children just like you, who lived so long ago that there were no cars, no roads, no supermarkets, no trains, no airplanes, no televisions." And so on, lots of examples of just how much time had passed since the chapel was built.
He must have got the idea, because eventually he leaned towards me and pointed at the vase of daffodils on the altar, saying in a hushed voice, "And mummy, look - those flowers are still alive "