Depends on what you define as "Old". Someone in their 30's is "Old" to someone that is 5! Generally the term "Died of old age" is for someone that is very old such as 90's or even in 100's. Also you...
Because once a year, they have a birthday which celebrates when they were born. On every birthday, you grow one year older. You age older and older until you die.
^^Don't lie