What are some roles for older children in the wedding?

Depends on the type of ceremony you're having, as in religious or not. If it's a catholic mass you could have them bring down the gifts, otherwise they could be ushers or hand out programs, or possibly escort VIP guests during the procession (such as grandmas).

When I was younger my cousins had me help cut the cake! It made me feel really special and was fairly easy

Depending on your ceremony they could performa reading, be ushers, hand out programs, walk down the aisle with a sign that says Hey Uncle so and so your Bride is coming....

I have four nephews and one niece. I wanted to incorporate all of them in the wedding by having the oldest (who will be 18) walk my mother down the aisle and serve as an usher, handing out programs and helping people to their seats before the wedding. The 1 and 3 year old will serve as my flower girl and ring bearer. The 7 and 8 year old, I wasn't sure what to do with, but we have decided to paint signs for them to carry down the aisle (after the FB and RB) that say "Please Rise" and "Here Comes The Bride."

With the ages of the kids you mentioned, I think that asking them to help with the guestbook (if you have one) or asking them to be ushers would be nice. It would give them a sense of responsibility, but wouldn't be asking too much.

You could have a junior bridesmaid/ring bearer. Ushers, pass out programs, but ultimately you don't need to give every child a "role". Unless you really feel inclined then I'd just let them be guests with their parents. We were going to have our cousin pass out programs at our ceremony but we ultimately decided not to buy programs.