A/somebody’s second childhood
Definition 1 of 2
Pro Tip 1/3Age-Related Usage
Used to describe elder people showing childlike behavior; not to denote actual childhood. 
Since grandpa's health declined, he's been in his second childhood, forgetting names occasionally.
Pro Tip 2/3Sensitive Context
Use carefully; it could sound disrespectful, as it involves mentioning someone's mental state. 
Aunt May is having moments of second childhood, so we need to be patient with her.
Pro Tip 3/3Not Literal
It's metaphorical; doesn't mean the person actually becomes a child again. 
Sometimes, when he forgets where he is, it's like he's in his second childhood.