The oldest Tuck boy, Miles, had married and had children when his wife realized he was not aging. 

“I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her" (Ch. 7).

This was very sad for Miles, of course. He had done nothing wrong, and he lost his family. Jesse never married, because he was too young when he drank from the spring. As a result of the spring water, Miles and Jesse could never have families. All of the Tucks keep separate from most other people so that no one will notice they are different. It is a lonely life.

"It was the same with our friends," said Mae. "They come to pull back from us. There was talk about witchcraft. Black magic. Well, you can't hardly blame them, but finally we had to leave the farm. We didn't know where to go. We started back the way we come, just wandering” (Ch. 7).

The Tucks only stay in one place for a decade or so, and then leave. Even while they live somewhere, they avoid spending too much time in town. They don’t want people to notice their inability to age. The little house by the lake that Winnie admires so much is just a temporary home. Mae Tuck tells Winnie they will need to be moving on from there, too.

Miles' wife had left him because after they had been together for a number of years, she realized that oddly, he was not aging. This was just too weird for her to handle, and, thinking that he had done something unnatural, such as perhaps sold his soul to the Devil, she left him, taking their two children along with her (Chapter 7).

At the time Miles' wife had left him, the Tucks did not know what the magic waters had done to them. They only knew that strangely, nothing seemed to be able to hurt them, and that while their friends and acquaintances were aging normally, they themselves were not. By the time they figured out that they had somehow been rendered incapable of growing old and dying because of the supernatural powers of the magic spring, it was too late for Miles and his wife. Miles, who should have been in his forties, still looked like he was twenty-two, not too much older than his own children.

When the Tucks figured out about the powers of the magic spring, Miles considered going out and searching for his wife and children so that they could drink the water and live forever too, but after thinking about it, he had decided that it would be no use. His wife would have been nearly forty by then, and his children grown, while he was still stuck in his early twenties. As Miles explains, "it'd all have been so mixed-up and peculiar, it just wouldn't have worked," so he just left it alone. When Miles tells Winnie about his family, he reflects that his daughter, whose name was Anna, would "be close to eighty now, if she's even still alive." His son, who is older, would be eighty-two" (Chapter 17).

Miles' situation illustrates the truth behind Pa Tuck's insistence that the powers of the magic spring be kept secret. Although the idea of living forever might seem appealing at first, the reality is that it would remove the recipient of this benefit from the natural stream of life, resulting in confusion and ultimate isolation.

Miles’s wife left him because he was not able to age, and she was.

When the Tuck family first drank from the magic water, they did not realize it would make them immortal.  As a result, they lived normal lives for a while before realizing they were not aging.  His wife could not handle it, and she left.  Miles explained that he thought about looking his children up, but it would do more harm than good.

They’d have had a pa close to the same age they was.  No, it’d all have been so mixed up and peculiar, it just wouldn’t have worked …  (p. 84)

Miles understands why she left.  Miles does not stay away out of lack of caring.  He is just trying to save himself and his family the pain that it would cause to know that he will never age, and they will.

That part of Miles's past is explained in chapter 7.  The Tuck family is explaining to Winnie how they came to know that they were immortal.  The Tuck family explained that they began noticing that they weren't getting hurt the way that they should be.  Angus Tuck was bitten by a snake and no problems.  Jesse fell out of a tree, landed on his head, and walked away.  Miles was even shot, but the bullet left almost no mark.  As time continued to pass though, the Tucks began noticing that the world around them was changing, but they were not changing.  

But it was the passage of time that worried them most. They had worked the farm, settled down, made friends. But after ten years, then twenty, they had to face the fact that there was something terribly wrong. None of them was getting any older.

Miles then gives Winnie details about his wife and kids.  

"I was more'n forty by then," said Miles sadly. "I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her."

Miles's wife left him, because she was scared.  She doesn't know why he isn't aging, but she knows something is definitely wrong with her husband.  She blames it on Devil worship, but it really doesn't matter what the cause is.  He creeps her out.  It makes sense.  I've been married for eleven years.  If I looked the same now as I did when I was 23, my wife would be freaked out by me too.  If she wasn't scared, then for sure she would be angry with me for not looking any older.  

Miles's wife was not alone in her fear of the Tuck family though.  Their friends and neighbors began to be suspicious and scared too.  The Tuck family was forced to eventually flee.  

"It was the same with our friends," said Mae. "They come to pull back from us. There was talk about witchcraft. Black magic. Well, you can't hardly blame them, but finally we had to leave the farm.