Perhaps 18 months is just right. We were actually looking at a fish scale. It looked similar to the photos below. The microscope is a toy one I picked because it has an LED and was die cast. It claims 1200X. The chromatic aberration is terrible! Also, when you are at the maximum magnification, oh how I wish I had an X-Y slide for the table.
He's not actually using the microscope. Just imitating me and his sisters.