I would definitely advise against this. Tell them kindly, either as someone else suggested that your health is not up to it, or say that you would need to charge them rent and housekeeping money and that you cannot ascertain how that would affect your pension and income tax. You can also truthfully say that you are not certain that your household insurance would cover anything that was damaged or went missing either of your or their property.
Actually, you are under no obligation to state your reasons for saying no. I wouldn't myself, as I would regard it as an imposition that casual friends would even make the request. Just say, you are sorry, but you cannot help.