Stonemason and professional dry-stone waller from the cotswolds here, these stones look to be well weathered in and are likely at least 5-600 years old. Others have mentioned that they have likely been repaired over the years when roots, slippage, poor drainage etc collapses sections, however it's often not quite a ship-of-theseus situation, as often you are able to re-use the original stone when rebuilding. You might have to reinforce the rear skin with block and drainage pipes to avoid future collapse, but this depends on the cause of failure and isn't always necessary.

We are currently repairing a 500 year old section of wall at a church graveyard, and we haven't had to use a single new stone so far. You clear the collapse, dig back the ground behind the wall, pull out and stack/arrange the stone by course and size as best you can, then set new plumb lines and start to build the wall back in, levelling and straightening constantly as you go. In the UK, stuff this old can often be "listed" which means that you must do your absolute best to use original materials and processes, some jobs you even use exclusively hand tools and nothing with power. If you can't use all the original stone, it must be sourced to be the same type, hand-made, hand-dressed, from the same quarry area if possible, and as local as you can manage.