@Kaarme
After this chapter, we can in fact believe the boy in her memories should be around middle/high school, since the random guy caught her attention.
When she bumped into the kid that reminded her of Sadao, she commented that it looked like
his middle school uniform. Meaning Sadao was already in middle school...
when she was still a grandmother. And, she's now, what, around 5 years old (I can't recall if it's stated how old she currently is now, but she's definitely around that age)? Meaning that even if she was reincarnated literally right after she died her grandson could easily be about or already have just graduated high school in Japan (middle school in Japan goes from about no less than 12 to around 15 on average). Meaning that even if Sadao was just entering middle school as she died, he'd still be around 17 or older even at the
youngest
The problem is she's already a little girl. That's compounded time added to however long since she's died. So even if it was an immediate reincarnation... she still had to wait until she was old enough to walk and talk in her new body before she could do anything. And, strangely enough, she put it off until after that, even. I mean, when she's this old and already in pre-school, I'd have figured she'd have already checked up on her past life's family at this point.