Lack of hygiene. Most common things that I see, poor oral hygiene, infrequent visits to the dentist, poor dietary habits such as eating habits, soda, coffee with sweateners, (don't sip sugar drinks for a long time, drink and be done), timing of sugar intake. A lot of times parents neglect themselves for their children which can be problematic (though I understand). Of course tobacco products and alcohol add.
ORAL PROBLEMS. Older people have teeth problems. Most of them have dentures or no teeth at all. Some cannot afford to go to the dentist that's why they end up with oral health problems.
Health changes. As you age, immunity becomes more precarious, many of us accumulate health conditions and or medications that slow healing. A lifetime of wear & tear on your oral tissues (teeth & gums) can also leave you more vulnerable to decay and gum disease. Details are specific to every individual and it is wise to consult with both your dentist and you pcp to manage specific risks as they relate to you.