Most people, no matter their age, should have a Durable Power of Attorney. This power of attorney lets someone (your agent) manage your financial affairs simultaneously with you. Depending on where you live (I'm only familar with Texas), this power of attorney may be designed to go into effect immediately or not until you are unable to make decisions (a springing power of attorney). In Texas, you may make the decision as to when your power of attorney goes into effect. However, if you do not trust the person you designate to have this power while you are able to manage your affairs, why would you let them do so if you are unable to manage your financial affairs? Also, with a springing power of attorney, there must be additional paperwork to accompany it to show that you are incapacitated (a doctor's statement, etc.). This makes it less likely that the power of attorney will be honored. In Texas, financial institutions sometimes refuse to honor such documents. My preference is a power of attorney that goes into effect immediately.
If it is likely that someone will have to manage your affairs, then it may be better to give your agent signature rights on your financial accounts. This is especially true if the agent is a close relative. Even if you do this, you still need the durable power of attorney. The term durable is used to indicate that within this document there is a statement that says that the power of attorney remains in effect even if you are incapacitated.
A copy of this document should be kept with a copy of your medical power of attorney to take with you if you are hospitalized. In some states, these two documents may be combined into one. The durable power of attorney may be needed so your agent can sign admitting documents, etc. Remember the durable power of attorney is only effective when you are alive.
Because a durable power of attorney is so powerful, I recommend that you discuss its implications with an attorney before executing one. If you choose to do one without an attorney, then just remember that anything you can do with your financial resources, your agent can do, too.