I've heard that you're never too old for trick or treating.
I've seen many 20 to 30 year olds out Trick or Treating but I personally stopped Trick Or Treating once I turned 13 because in my opinion 13 and above is too old.
I personally felt too old at 13 and above to be Trick Or Treating and that's just my opinion.
Also be aware that some cities ban people 16 and above from actually Trick Or Treating due to Vandals so you could potentially get into trouble although that's rare.