My fence between my house and my neigbour is the usual lap fencing panels with posts held in the ground by metal stakes, the ones you drive in.
I think the life span under normal contitions is 15 years.
My neigbour has leaned some of his old large heavier fence panels up against my fence.
If after I have told him the possible outcome seeing my posts have been in the ground for 15+ years and my fence collapses is he liable for any damage it may cause.
WHen you use the word 'liable' it suggests you would have some legal recourse to either prosecute him or gain some financial compensation. Is this your aim?
If the fence falls down whilst your neighbours stuff is leaning on it - then you would have a claim. However, you'd only be able to claim the value of a 15 year old fence - ie not new-for-old.
The usual way to calculate this would be to divide the life expectancy by the age - ie a fence thaat should last 15 years, is worth one third of its original cost after 10 years. By this calculation, your fence has almost no value.