As a result of his fourteen years in captivity, Dr. Manette has, indeed, aged. In addition, he has also developed a certain disorientation with time and circumstance. For instance, when Mr. Lorry and Lucie Manette arrive, he does not comprehend that he "has a visitor." He looks up momentarily, then resumes his habitual action of shoemaking. Dickens writes,
The task of recalling him from the vacancy into which he always sank when he had spoken, was like recalling some very weak person from a swoon, or endeavouring, in the hope of some disclosure, to stay the spirit of a fast-dying man.
When Manette does commit some action, in the "midst of the action he went astray." His mind wanders into the vacancy in which it has been kept so long. After he is brought back to London and lives with his loving daughter, Charles Darnay wishes to ask permission to marry Lucie. In an attempt to be forthright Darnay begins to reveal his true identity, but Manette stays him, telling Darnay to reveal his secret on the wedding day. When Darnay does so, poor Dr. Manette pulls out the shoemaking tools and recommences his habitual task. As he does so, he again becomes disorientated, losing a sense of time:
He had laid aside his coat and wastcoat; his shirt was open at the throat...and even the old haggard, faded surface of face had come back to him.
When Mr. Lorry tries to pull him back to reality, asking him to look at him, Manette does so, but
in the old mechanically submissive manner, without pausing in his work.
Manette continues working on the shoe that, he says, "ought to have been finished long ago." Even later in the narrative, when Manette gets better and bravely travels to London to plea on behalf of his son-in-law, Charles Evremonde, he does not grasp the true nature of the situation in Paris. Truly, Dr. Manette's experience of being imprisoned for fourteen years has left its mark upon him in A Tale of Two Cities.
I can't tell where you are in your reading of A Tale of Two Cities, but if you've met Dr. Manette you already know he's a man in deep distress when Lucie and Jarvis see him for the first time above the Defarges' wine shop. Physically, his hair has gone white and he looks much older than he is. It's the emotional and psychological effects of his time in prison which are much more disturbing and dramatic, though, and we get his story in bits and pieces throughout the novel.
When he is in his "prisoner" mode, he doesn't make eye contact with anyone in the room; he keeps repeating a number which seems to be an address--one hundred five North Tower. He doesn't recognizeanyone else in the room (which is understandable at first, since he didn't ever know he had a daughter and hadn't seen Jarvis for years). All of these are pictures of a man who is broken and somehow humiliated or broken. Most importantly, though, during these episodes he reverts to the pastime he had while in prison--making shoes.
"Making shoes" comes to be akind of code word for what happens when Dr. Manette has reverted to his prison days. We learn he asked for something to do to pass his time in prison so he would not go crazy. The physician became a shoe-maker to keep his sanity. He keeps his head down, he doesn't appear to recognize anyone around him, he is unaware of his surroundings, and he works non-stop at the business of making shoes. It is intricate work, and he uses it to keep his mind occupied and off whatever troubles are worrying him.
When Dr. Manette reverts to his shoe-making ways, it is because something has caused him emotional distress--even the good kind, such as his daughter's wedding.
In Ch.5 of Book I Defarge leads Mr.Jarvis Lorry to Dr. Manette's room upstairs in a rundown house in a seedy locality in Paris. As they draw near the room Defarge, to Lorry's surprise, draws out a key and Lorry asks,
"The door is locked then, my friend?" said Mr. Lorry, surprised.
"Ay. Yes," was the grim reply of Monsieur Defarge.
"You think it necessary to keep the unfortunate gentleman so retired?"
"I think it necessary to turn the key." Monsieur Defarge whispered it closer in his ear, and frowned heavily.
"Why! Because he has lived so long, locked up, that he would be frightened- rave- tear himself to pieces- die- come to I know not what harm- if his door was left open."
"Is it possible!" exclaimed Mr. Lorry.
"Is it possible!" repeated Defarge, bitterly.
This clearly prepares the readers as to what to expect as soon as they see Dr.Manette for the first time in the next chapter, aptly entitled: "The Shoemaker."
Dickens conveys to his readers the extent to which Dr. Manette has been dehmuanized not so much by the physical description but by the description of the feebleness of his voice:
The faintness of the voice was pitiable and dreadful. It was not the faintness of physical weakness, though confinement and hard fare no doubt had their part in it. Its deplorable peculiarity was, that it was the faintness of solitude and disuse. It was like the last feeble echo of a sound made long and long ago. So entirely had it lost the life and resonance of the human voice, that it affected the senses like a once beautiful colour faded away into a poor weak stain. So sunken and suppressed it was, that it was like a voice underground. So expressive it was, of a hopeless and lost creature, that a famished traveller, wearied out by lonely wandering in a wilderness, would have remembered home and friends in such a tone before lying down to die.