ERASED #01-08, known in Japan as Boku dake ga Inai Machi (僕だけがいない街?, lit. The Town Without Me)
The story follows Satoru Fujinuma, a man who somehow possesses an ability that sends him back in time moments before a life-threatening incident, allowing him to prevent it from happening. When his mother is murdered by an unknown assailant, Satoru's ability suddenly sends him back eighteen years to when he was still in elementary school, giving him the opportunity to prevent a kidnapping incident that took the lives of three of his childhood friends, two classmates and one young girl studying at a school nearby
It definitely borders on some aspects of the age regression fetish but it's never a complete focus of the show. For example, Satoru will sometimes slip into his childhood persona much like how AR writers treat mental regression and he will even recognize he's doing as such, but it never goes beyond this. It's meant to be cute and endearing for the most part, there's no threat of him completely becoming an elementary school boy unable to solve the mister of his mom's murder.
All that said it's a really, really good anime and definitely a great as well as a serious application of age regression for a mainstream audience (at least of those who like anime and manga)