Technically, living will is just a part of advance directives that any person could take to describe and specify medical treatment preferences in case of end-of-life occurrences. Such end-of-life events could occur at any age. In general, adults need a living will more if they want to decide any medical procedure or treatment to take before the need arises.
To simply put it, a living will is describing preferences with regards to any treatment in case a person is subjected to any serious illness or accident. The legal document would speak for that person in the moment he becomes practically unable to express or speak for himself, like in the case of a coma. As such, a living will is not just for adults, as mentioned. Legally, any person who is over the age of 18 years could appropriately prepare living will and other legal advance directives.
In definition, a living will, and all other legal advance directives, is a written instruction about a persons specific medical care preferences and choices. If you make one, your family and your doctor would automatically consult the document in case you become unable to decide for yourself regarding significant medical treatments and procedures. Take note that the document could be drafted and prepared by you, but there should be legal or lawyers assistance and presence to make it valid and binding. It may not be as important as a will or a living trust, but more and more people nowadays are deciding to have one, with regards to high medical costs.
Any living will could also include a medical POA or Power of Attorney and a DNR or Do Not Resuscitate order. Some people prefer or miss out unintentionally to include these two. In many cases, inclusion of any of the two has proven to be advantageous to all concerned parties.
The medical power of attorney of medical POA is a document (legal) that designates an individual (also called a healthcare proxy or agent) to carry over or make important medical decision in case the person getting the medical POA becomes unable to make that decision. This is also called by some as the durable power of attorney for healthcare.
Be informed that medical POA is very different from the usual power or attorney that is used to authorize any lawyer to takeover financial transactions for a client in specific cases. Many living will are now including medical POA, especially when people owning the documents aim to spare their family from making difficult and heart-breaking medical decisions in the future.
On the other hand, the DNR order or Do Not Resuscitate order is a special request by a person not to take any cardiopulmonary resuscitation if the heart suddenly stops beating or breathing is ceased. A living will could or could not include a DNR order. The DNR order could also stand alone in itself and may not need any living will or advance directive to be effective and implemented. Thus, any legal procedure could be waived.
A persons medical doctor could indicate a DNR order in his medical chart (of course upon the wish or request of that person).