My Ped is pretty strict about switching to 2% (or less) at 2 years of age - he says after 2 years they don't need the full fat anymore (which they previously needed for brain development) and it's a cardiac issue - easier to switch them when they are young to help avoid heart problems later. His only exception is for low weight kids, etc. It's pretty standard for kids not to gain much, if any weight, from 18-24 months so as long as Katie is a good eater and good weight, IMO she'd be better off switching.