If your lucky you may have been able to trick or treat up to the age of 13. After that it becomes socially unacceptable to go house to house begging for candy from strangers. I remember as a young child when I first heard that there was an age limit to trick or treating I was filled with fear. To a eight year old, 13 did not seem that far off. I was terrified for the day I wouldn't be able to get free candy that would last me until the next holiday, Thanksgiving.
For all non-children, Halloween is not always celebrated to its fullest extent. Similar to Valentines Day; you only celebrate if you have a reason to. You aren't even allowed to dress up for school after elementary school. While by the time most people reach high school they no longer see the appeal of Halloween (might as well just trick or treat to Walgreen's and be done with it) it seems that the importance of Halloween diminishes as you go older. Maybe it's partially because most of the time Halloween falls on a school night, or it could be that teens have enough money that they can go out and buy themselves candy instead of having to walk blocks to accumulate a decent stash, or maybe teens simply just don't want to dress up.
While many teens and young adults still have Halloween spirit, it seems like many people simply view Halloween as being one step closer to Christmas.