Our son is 7-yrs-old and it fits him perfectly right now...he is a little taller than his peers but he will probably grow out of it in the next several months.  The costume size "M" says it fits 8-10 years old but there is no way this will fit him by the time he turns 8 in August.  The mas is also a little small and doesn't even cover well!  BUT the boy loves it!