Why is my older dog peeing on the carpet?

shes really old. shes 10, and shes a golden retriever. she was my nans dog untill my nan ped and we took her. we had her for about 3 years and shes never done this. we also have a 3 year old australian sheperd. but we had my aussie for a while, and shes not very mean to my golden. ( anymore ) when we first got her tasha went crazy, always bitting her, never letting her do anything, she was so jealous. but now tashas fine with her. i dont get it. brittany ( the golden ) cant be stressed nothings changed? we havent done anything that would be stressfull to her, She just all of a sudden started doing it, and my parents are getting really pissed off. This is happining to my friend too. she has a very old dog named lucky whos 12, and they have a younger dog named bella, whos a puppy. but she never trys to play with lucky, so he cant be stressed either? help?