Regarding Halloween trick or treating, so long as you have a costume I feel you can go out. Sadly, many people see someone as 'too old' and won't give them candy. [My one friends mom doesn't give candy out to anyone over 10] it's a fun thing to do and you should be able to do it until you feel you've grown out of it. But Halloween is more than just 'trick or treating' have you read into the history? Trick or treating isn't all that old. You could make new traditions like going to haunted houses or corn mazes. [I read The Legend of Sleepy Hollow every year] plus you can still dress up, eat candy, carve pumpkins, and do whatever else you like about the holiday.
Now as it's mentioned Samhain is a pagan holiday and many feel insulted by the way modern culture has hijacked it. I feel there's a way both can live in balance, but all these questions about how to celebrate as you grow up are things you should figure out. I loved trick or treating as a kid, in my teens I went out for free candy then had an all night Samhain celebration with my coven, and now I just celebrate Samhain. [Of course decorating the house and dressing up is always fun, and if you want candy, go to the store November 1, discount candy day is adult trick or treating lol]