What is a lifetime guarantee?

My Junior year in high school I was on the swim team and everyday after practice there was four of us that would sit int the sauna and talk. One day the question of what is a lifetime guarantee came up. None of us knew what the right answer was but we did come up with a few different options for what a lifetime guarantee could be. The first option is that a lifetime guarantee would be for the lifetime of the product which would be pretty pointless because that would mean that when the product breaks or stops working then it wouldn't be guaranteed anymore so you couldn't get it replaced. Another option was that it was guaranteed for the lifetime of the person who purchased the product and if the is the case is it no longer guaranteed when I die or if I purchase the product when I am twenty and I die when I am ninety is the product still guaranteed for 20 years after I die since I only had it for 70 years and my lifetime is actually 90 years long? I suddenly thought of this of conversation last night that we had 4 years ago and we have still not come up with the correct answer. Honestly I don't even know if there is a right answer but I thought that I would write a post about it to see what other people think.