It is still a child. I don’t think it translates
1:1 with human aging, but this species has a particularly long infancy-early childhood stage.
There might be something unique to the Yoda species regarding Force sensitivity, but it’s already been established in canon that Force-sensitive babies in general have a predisposition to using the Force, regardless of species. The idea being, I’m assuming, that babies and young children lack some of the qualities that adults have that can hold back their ability to use the Force (like doubt or disbelief). And it is possible that the time he spends in this young childhood stage plays a part in why he can lift a mudhorn vs a toy ball.