My guess would have been around 5 or 6 years old.
It's a great drawing but the tell-tale signs for me that it was a younger child were that:
a) it's a copied picture
b) the colouring-in is outside the lines
c) the drawn lines are wobbly and Pikachu's front legs (paws?) don't have any features - older children tend to start adding more detailed features like fingers, claws etc
None of that is negative - it's what you'd expect to see in a drawing from a younger child. At first glance it does look as if it's been done by an older child, but the signs are there that it's been done by someone younger who's good at art.
I want to emphasise she's clearly really good for a 5-year old and she loves it, which is even more important! Keep encouraging her OP - creative activities are wonderful for the developing brain and it's a great passion for children to have.
You can tell your DD from us OP that we all think that she's drawn a lovely picture!