Hi M.,
As a mother of 4 and grandmother of 5, I understand your dilema. It's difficult when our children don't want somehting we want for them.
As to whether or not 16 months is too young for trick or treating, I think the real answer is not the age of the child, but the purpose of taking him out for the event.
I know there are arguments on both sides of the camp as to whether or not parents should allow their children to celebrate Hallowe'en..there are pro's and cons on both sides. I am more concerned about the overall effect of the nights events. Dressing up and going out to get free candy can be fun, but what message does it teach? If you choose to TAKE (not send) your kids trick or treating, be sure to advise them of the importance of going to "known" homes, and other safety precautions.
Many cities now have "safe" trick or treat outings at their local businesses, as well as many churches offer alternative activities. I encourage you to listen to your 16 month olds objection to the costume, and maybe make a simple cat or clown face out of make-up for him instead. You want this event to be fun for him. You also want to allow him to be wary of strangers.. It's a built in safety mechanism.
I hope this has been helpful
Grandma A.