The rules on when you can start activating a lasting power of attorney vary depending on what type of LPA it is.
If it’s a health and welfare LPA, you can only activate it if the donor (that’s the person who made the LPA) has lost mental capacity and can’t make their own decisions.
If it’s a property and financial LPA, you may be able to activate it as soon as it’s registered. The LPA will say whether this is the case. If not, then again you can only use it if the donor is unable to make decisions on their own.
You can find out how you can tell if someone has the mental capacity to make decisions here.