My 18 month old is still using the same car seat that she's had since birth, rear-facing. she is still comfortable. what/when is the next step?

New seat: If she is over 20 pounds you need to check the weight limits of the chair...Usually by now the child should be facing forward...

Check the stickers: I am encouraged that you have her rear facing as many aren't that smart. I encourage mom's to keep their kids rear facing until at least 2 (prefer 3) or the upper weight limit of a convertible seat. Newer models go rf to 40 lb and then function ff to 50.The stickers on the seat should tell you what the limits are, if no labels, get a new seat. Keep up the good work.