Would these fit a 5 year old?

Not likely. It fit me and I’m almost 40 and wear a 7 3/8s hat. I used a pool noodle cut in a ring stuffed inside for our 1 year old to wear. It actually helped the hat stand up and look better too.

Would this fit an 8 year old or adult?

I bought this hat last Halloween it's to big for my 4-5 year old but my son who is almost 9 fits it , it's just bigger then the hats you find at Walmart with the costumes for toddlers. I tried to share a photo but I can't log back into to try and do so for you .

Would this hat fit a one year old?

This hat may be too big for a one year old.

What is the diameter of the part of the hat that fits on the head? Not the puffy part that sticks out to the sides.

Hi there, The diameter of the Luigi hat (the inside, the part the kid put on his head) is about 7.5 inch. However, the back part of the hat stretch a little more to expand. Let us know if you have any other question we will be glad to answer :)