"What must have been," he thought, "the youth of those extraordinary men, who, even as age is stealing fast upon them, still are able to conceive such plans, and to carry them out without flinching!" (22.2)
Here youth is glorified. Fouquet could have understood the men as benefiting from experience that old age gives, but instead chooses to imagine their youth as even more powerful.
This last endearment was tender as in youth, as in times when the heart was warm and life happy; and then Porthos mounted his horse. (26.63)
In contrast to aging, youth is seen as the highlight of one's life.
"Alas! we are no longer the young invincibles of former days. Who knows whether the hatchet or the iron bar of this miserable coaster has not succeeded in doing that which the best blades of Europe, balls, and bullets have not been able to do in forty years?" (31.28)
This is one of the first acknowledgments of the four friends' mortality.
"I am strong against everything, except against the death of those I love. For that only there is no remedy. He who dies, gains; he who sees others die, loses. No; this it is,- to know that I should no more meet upon earth him whom I now behold with joy; to know that there would nowhere be a d'Artagnan any more, nowhere again be a Raoul,- oh! I am old, see you, I have no longer courage. I pray God to spare me in my weakness; but if he struck me so plainly and in that fashion, I should curse him. A Christian gentleman ought not to curse his God, d'Artagnan; it is quite enough to have cursed his King!" (32.69)
Yet another testament to the fact that life becomes increasingly complicated when one ages, and approaches certain death
"I believe in the times in which you live men are more engaged in their own interests and their own pleasures than they were in our times. You have sought a secluded life; that is a great happiness, but you have lost your strength in it. We four, more weaned from these delicate abstractions which constitute your joy,- we found in ourselves much greater powers of resistance when misfortune came." (33.36)
This statement reflects Athos's age.
She praised him; she blamed him; she bewildered him. She showed him the inside of so many secrets that for a moment Colbert feared he must have to do with the devil. She proved to him that she held in her hand the Colbert of to-day, as she had held the Fouquet of yesterday. (36.4)
Madame de Chevreuse has not missed a beat with age. It has only made her tougher.
Age, which had been kept back by the presence of the beloved object, arrived with that cortege of pains and inconveniences which increases in proportion as its coming is delayed. Athos had no longer his son's presence to incite him to walk firmly, with his head erect, as a good example; he had no longer in those brilliant eyes of the young man an ever-ardent focus at which to rekindle the fire of his looks. [,,,] The Comte de la Fere, who had remained a young man up to his sixty-second year; the warrior who had preserved his strength in spite of fatigues, his freshness of mind in spite of misfortune, his mild serenity of soul and body in spite of Milady, in spite of Mazarin, in spite of La Valliere,- Athos had become an old man in a week from the moment at which he had lost the support of his latter youth. (41.1)
Having a kid may have kept Athos young. We would also like to point out here that Raoul is Athos's one weakness.
"Come, let us see what stops you?" said the King, kindly. "You have given in your resignation; shall I refuse to accept it? I admit that it may be hard for an old captain to recover his good-humor."
"Oh!" replied d'Artagnan, in a melancholy tone, "that is not my most serious care. I hesitate to take back my resignation because I am old in comparison with you, and I have habits difficult to abandon. Henceforward, you must have courtiers who know how to amuse you,- madmen who will get themselves killed to carry out what you call your great works. Great they will be, I feel; but if by chance I should not think them so?" (53.64)
This is an issue that transcends old age – the King is essentially asking D'Artagnan to stop thinking for himself.
"When will it be my turn to depart?" said he, in an agitated voice. "What is there left for man after youth, after love, after glory, after friendship, after strength, after riches?" (60.36)
This is D'Artagnan's first time in the novel reflecting upon his inevitable death. Also, old age is clearly not being appreciated. After youth, love, glory, friendship, strength, and riches, D'Artagnan implies that it is time to die.
"The Duc d'Alameda?" said d'Artagnan, reflecting in vain.
"I!" said an old man, white as snow, sitting bent in his carriage, which he caused to be thrown open to make room for the Musketeer.
"Aramis!" cried d'Artagnan, struck with stupor. And, inert as he was, he suffered the thin arm of the old nobleman to rest trembling on his neck. (Epilogue.61 – Epilogue.62)
Although they are the only two out of the four Musketeers left alive, they have not bothered to maintain their former closeness. Old age may have ravaged the friendship.