Please, take them to the vet and get x-rays done before assuming that it is simply age.

I made this mistake with my rottweiler. She was slow getting up a couple of mornings the beginning of Feb. 2005. About a month later, she couldn't put any weight on her front leg one afternoon. The decision to put her to sleep was made later that night when the x-rays showed the very advanced osteosarcoma in her shoulder.