You should have a will, DPA and PAD if you are 18 years old or older, if you have or are expecting children, or if there has been a recent death in your family.
Having these documents helps ensure that should something happen to you, your family and friends will not be left in the dark regarding your wishes. It also helps provide for the care of your financial assets, and puts decisions regarding your medical treatment in the hands of a person of your choice, who has been informed of your wishes.
A will is a legal document that mandates certain actions after your death -- specifically, the disposition of your assets. You can also designate guardians for your minor children (under 18 years of age), distribute your property/possessions, appoint an executor, and other important tasks that must be handled upon your death.
If you do not have a will, the state will usually take over your estate and distribute it according to its own rules -- which may not work with your particular family. It might also cost money, as court-appointed executors might charge a fee, and your estate could be tied up for years while your relatives argue about the division.
A Durable Power of Attorney (DPA) is a legal document that appoints another person to act as your 'attorney-in-fact' (in this case, 'attorney' does not necessarily mean a lawyer; rather any person authorized to act on another's behalf). Should you become unable to act on your own behalf (disabled, mentally incapacitated, etc), the person appointed your attorney-in-fact will have legal authority to act on your behalf. You can give them as much or as little power as you wish. However, they are not authorized to make medical decisions unless you specifically designate them as your Patient Advocate as well as your attorney-in-fact.
A Patient Advocate Designation (PAD) appoints a person to make important medical and health-care decisions on behalf of an incapacitated person. This document is drawn up in advance and takes effect upon a health care professional's declaration that a patient is incapacitated and unable to make their own health-care decisions.