Hedgerow dating
Hedges can be very old features. In particular, many of the hedges separating fields from lanes in England date back to the medieval period. The late Max Hooper developed a dating tool which relied on the discovery that woody trees and shrubs colonise a 30m run of hedge roughly once every 110 years.
It is only a "rule of thumb", e.g. the inset shows a chestnut seedling at the base of the hedge. Clearly that doesn't make that section 100 years older than the rest of the hedge. On the other hand, it is conceivable that it might take a fair length of time to grow, fruit & colonise the entire hedge. The hedge alongside the orchard has an average of 6.7 woody species per 30m stretch, implying that it has a medieval origin. If so it would be the oldest feature in the garden.
More on hedges at https://en.wikipedia.org/wiki/Hedge