The whole diet and exercise is everything is partly true but genetics can take you for a bad turn or maybe your work history. No one will argue that working outdoors in construction is "exercise" but it will also tear up your body like nothing else. Throw in some genetic conditions like RA, osteo etc and you can have issues regardless of what you do.
You simply have to deal with life and the cards you are dealt. Some can be active into their 80s and 90's even while others can be a shell of a person in their late 60's with Alzheimers.
Enjoy the time you have and get to the best of your ability and try and not hurt yourself by being over weight or doing detrimental things to your health.