I ordered this costume for my baby who will be 10 months old for Halloween but already wears size 18m at 8 months old.  The description of the costume in the search results says it is size 2-4, but there are buttons that allow you to select size 0-6 months, 6-12 months, or 12-24 months.  I figured a size 12-24 months would be perfect for my baby this year, and probably still fit him next year.  What arrived was a size 2-4T costume.  This is okay, because I will be able to use it in the future, but it is not the size I thought I ordered so be aware when you are ordering that you may get a size 2-4T regardless of the size you specify.
The costume looks cute, and although it's too big for my baby this year it will be a fun costume next year.