Jiisan Baasan Wakagaeru

Both shows have a similar premise where someone in the family becomes younger. ('Reincarnation' being the main plot element for TsumaSho; whereas Jiisan Baasan has spiritual/supernatural elements). The common recurring theme in both shows is 'Second-hand Happiness'; where they proudly see their kin maturing becoming upstanding people as they face challenges in becoming an adult. And helping them face those challenges when needed.  

Both are more laid back comedy style shows with a bit of slice of life style content here and there. Both shows are very wholesome and enjoyable from start to finish with satisfying endings. The main differences are while one takes the aspect of a old couple turning young and enjoying the end of their lives feeling young with their families. The other takes 2 semi socially awkward high school students becoming friends, making new friends, playing games, and enjoying their high school lives. 

『Jiisan Baasan』& 『Mom, I'm Sorry!』are both heartfelt shows that's about the joy of becoming young again much to the surprise of their other family member(s) . Circumstances of certain characters are similar in both shows where the mother had a terminally ill condition which is cured again due to becoming young again. It also explores the conceptions of getting another chance at a life that was missed out due to the nature of their pasts and the circumstances surrounding their long forgotten /lost dreams. With their new-found youth they get that time to do things they couldn't do in their past. Jiisan Baasan has a more 4-koma style episodic approach as opposed to Mom, I'm Sorry's storyline which presents an overarching storyline. read more 

Both series have a loving elderly couple as main characters Both series are slice of life