What is your relation, if any, to the person in question? Are you a family member or a close relative? Can you establish your intention of good will concerning this person (and not just the fact that you are financially "interested")? What are your "credentials" in terms of money management?
I have seen two cases of this, and in both cases the person who tried to procure the status of 'power of attorney' "bought off" lawyers and medical people with bribes to endorse the procedure. One got caught at his game and the other didn't (but time will tell.)
If your motives are truly honest and you have the best interests of the person in mind, then you needn't hesitate to contact a competent lawyer with ethical integrity to discuss your particular situation. It is difficult to answer such a general question since law differs so much from one country to another.
It goes without saying, though, that taking such initiative looks "crooked" from the outside, even if it isn't, and your motivation for taking such initiative will automatically be under close scrutiny.