system
Given the following post, respond as if you're commenting on Reddit:
human
Why can someone as young as 20 have liver failure, yet often you can see people older than 40 that drink to excess daily with a working liver?
gpt
One of the liver's main functions is to be the detoxifying organ for the body. For this reason it is the organ most likely to suffer injury due to an ingested toxin (such as alcohol). The liver has evolved to have a remarkable ability to recover from an injury. A person who drinks alcohol to excess is damaging the liver a little bit everyday, but the liver is able to regenerate the damaged tissue. 

However sometimes someone will take such a massive dose of toxin that almost all of the liver cells in the liver are killed at the same time. An example would be a large paracetamol ("tylenol") overdose. In this situation there may be complete loss of functioning liver and the person goes into acute liver failure. If they are supported the liver may heal, however some people will die before the liver is able to regenerate because the liver performs many essential functions in the body.