Sitting on the window sill of their combination study room / laboratory / library, Harry said, "Sir, I understand the concepts well enough. Through your magics and mastery of the knowledge of space and time you've created a piece of reality... space... outside of normal reality as the rest of us understand it. And, through your mastery of time, within that space you're able to manipulate time."
Harry had now been a student under Myrrdin's tutelage for what felt like a decade. But he did not really know because there did not seem to be any seasons other than Spring around their keep.
"Yes, you've got it so far. Keep going..." Myrrdin said, sitting at his desk.
"You're actually alive in the year 515 AD when you set all this..." said Harry, waving his hand about distractedly, "...up. And I'm alive in 1984 AD. At least, that was the year in which you pulled me out to join you here."
"Correct," the old man responded. "What else?"
"By creating this... pocket reality..." continued Harry pausing for a moment.
Interrupting, the old man said, "Pocket reality. I like that. It's a good name for it."
"...Pocket reality," continued Harry, "you've created an environment where we can both exist and you can teach me without causing a time paradox in... normal... reality."
"Correct. And what appears to be bothering you about this?"
"How does the paradox not occur when we drop out of the pocket reality back into normal reality?"
"Ah!" said the old man, light dawning. "I can return back to my own time in normal reality pretty much any time after I left. I'm far enough isolated in my little abode I rarely interact with anyone, or anything, going on. However, I'm planning on returning to my own time no more than a few seconds after I left, thereby not causing a paradox to a time before; nor upsetting the time continuum. People will not notice any aging effects upon my behaviour or body due to my advanced age; and I already know the role I'm to play.
"As for you, I can send you back to any time after I removed you from the Dursleys. However, I need to put you back into normal reality before the First of July 1991, when you're soon due to receive your letter from Hogwarts."
Harry sat for a little while gathering his thoughts before saying, "So, I've only a couple of years left before I have to go back. Can we get everything done in time?"
The old man grinned. "Think it through, lad. I can go back to my own time frame barely a few seconds after I left. And should, as I've already explained."
Harry nodded focussing on his teacher's words, "So that means time passing in here is irrelevant to time passing in normal reality."
"Correct."
"But what about the effects of aging in here, not just on my wisdom and knowledge, but on my physical and magical self?"
Instead of answering, Myrrdin asked, "You've not noticed you don't seem to be growing in line with children the same age as you in the time viewer?"
Harry shrugged, "I thought I was just small for my age. Probably due to the malnutrition I suffered at the hands of the Dursleys."
"No," replied Myrrdin. "I anticipated, in advance, it would take many years to impart upon you the knowledge you would need to defeat Riddle and counter Dumbledore's manipulations; and make a good start on restoring balance to the magical realm in Britain. As such I've slowed your aging process."
"I would have thought I'd learn faster if I was older physically, emotionally, spiritually and magically," said Harry.
"Yes, you would. Well done for recognising that," the old man congratulated him. "However, would you really want to go through puberty, in here with me?"
Harry visibly shuddered before answering with a grin, "No, no; definitely not."
He had not enjoyed the discussion about human sexuality he received from his mentor. As with all things, his mentor spoke with enthusiasm on the subject, and even used diagrams. He had trouble sleeping for a couple of nights after that particular lesson.
Grinning, the old man paused to ensure Harry had no further questions before continuing, "Now, shall we look once more upon the matter of the Protean Charm and its relationship with quantum entanglement?"
Harry groaned and dropped his head to his chest.
Myrrdin laughed and said, "Come now, lad. Since you've mastered Occlumency, and your mind is not the untidy clutter it once was, your memory is close enough to perfect to allow total recall at will. We only have to go over this once and ensure you understand, rather than just know it, before we can jump onto the next subject matter."
Knowing how to encourage his young student's eagerness for the subject at hand the old man said, "And after that, perhaps we shall make use of the time viewer for a while. Though, I still don't know why you insist on calling it by the acronym 'teevee'."
Harry sat bolt upright on hearing that with a big grin on his face.
"And, after that, can we work some more on magic?" the boy eagerly asked.
Chuckling, Myrrdin replied, "Of course we can. That's a wonderful idea!"