You an dress up and go trick or treating at any age as long as you're polite but most people stop trick or treating at the ages of 12 to 16.
I quit trick or treating at 12 years old myself but I do have some adults even come to my house on Halloween Trick Or Treating and I have no problem with it.
They're not doing any harm and are just out having fun and enjoying themselves even if they're alone without kids.
I have no problem giving them candy if they want to trick or treat although some people get embarrass and feel they're too old at age 12 or older including myself.