The choice of when to prepare a Power of Attorney document is completely personal. The decision should be based upon your comfort with the executor having powers over your assets, and your own capacity to make clear decisions.
If you currently need or want a family member or friend to have the executor powers set forth in the Power of Attorney document, you can and should execute it at once.
Or you can wait until a later time to execute and deliver the document, when you feel ready.
Final option, you can create the document but not deliver it to your executor; instead, leave instructions with the executor or another individual indicating that you have created a Power of Attorney document and that upon your disability, the Power of Attorney is to be utilized.