46 IS young for a grandpa. Though considering Ichika's gonna be a teenage mum it's not surprising her parents will be young grandparents.
That's based on Modern Norms, which promotes having late kids or no kids, so that companies & governments can earn extra from more income spenders
(aka., the women, who are only forced/included into the workforce to increase tax revenue and corporate sales).
Not by Village standards though, where people still get married under 20 atleast in Asian region. And if we consider the same for their parents, then the starting age to be a grandpa would be around 40.
On other hand, if we consider that its common for people at that village (in the story) to get married and have kids around mid-teenage years. Then the age to be a grandpa would be around 32-33 instead, at which point 46 is going to be a bit too old to be a grandpa.