The four Musketeers from Dumas' The Three Musketeers are all grown up, and The Man in the Iron Mask deeply reflects that fact. Everything is more complicated for our heroes than it used to be, meaning that old age is viewed negatively in the world of the novel. The glories of youth are over, so from here on out life is about finding a good way to die. Keep in mind also that our heroes aren't exactly old, per se. They're still vigorous and healthy men. Old age in The Man in the Iron Mask is therefore more a state of mind than a state of physical being.
In The Man in the Iron Mask, aging means that life becomes more complicated.
Youth, and not age, is celebrated in The Man in the Iron Mask.