2015 and earlier models seem to be having HV battery pack failures somewhat frequently... Especially the 85 packs from what I have seen. Perhaps represented disproportionately as everyone who has a problem asks about it versus probably thousands for every one of them without any issues.
This is a big ticket item around 15k USD depending on if you replace with a remanufactured or new pack.
Whether this is due to age or underlying design failure I do not know.
Please do familiarise yourself with the intricacies of SC01 code for free supercharging, this is a very important piece of information needed to ascertain whether you will inherit free supercharging for life or not. Additionally, Tesla is offering free supercharging transfer incentives to owners of older model S looking to purchase a new Tesla, beware of that complication as well.