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.
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More on hedges at https://en.wikipedia.org/wiki/Hedge
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Just across the stream, next to the road, there are a hedges that may go back even further. It is purely conjectural but across the country there are hedgerows that were started in Roman times. They are sometimes called "celtic" hedges.
Typically they enclosed long narrow fields and ran in a roughly north to south direction. A possible reason for the orientation is that if they ran east to west they would have acted as wind-tunnels for our prevailing westerly winds.
The hedge on the right in the Google image below would have continued along the line of large oaks and lined up with the old hedge in the garden described above
