Can I turn a mesh into a shapekey?

I have an adult character and I make a child version of her. I didn’t know I’d need a teenage version of her as well. Is there a way I can make the child version of her into the shapekey of the adult?

Your question is not clear, could you please give details?

I have two different models that are the same person at different ages. One of them is 20, one of them is 12. I want to combine the 12 yo model into the 20yo model, but I want to use a shape key slider to adjust her age. ShapeKey value 1.0 being her 20yo self and ShapeKey value 0.0 being her 10yo self.

That make sense?

there’s a way to morph from a mesh to another but you need to model the second one from the first one so that it recognize which vertex is which, and you can’t add any new vertex. To do so, duplicate your first object, model the second, then select the first object, shift select the second, and in the Shapekeys dropdown menu (down arrow key on the right of the shapekeys list) choose Join As Shapes. Or you could simply recreate the second shapekey within the same object.