My daughter is 5 years old, she finished the full game in roughly 2 x 1 hour.
Of course she does not understand fully what's behind (I would say almost "fortunately" ;-) ), especially on divisions or multiplications, so it cannot replace the future explanations about what it means.
That said, it shows the kind of game maths are about, and it dedramatize maths completely, and maybe that's the more important aspect of this game after all : maths are just a game, a complex one, but you have to enjoy them if you want to be good at them in the end.
In the end, the game is very progressive, fun, she loves seeing the little monsters growing and it entices her to go on. The UI is not perfect, but it's already very good. For example, giving some time in the end to cleanup extra members would be cool instead of relying on the order you remove members.
I can't help but think that playing complex games like this is one of the best training for yound minds !