Is it appropriate for a teenager to lead a committee of adults?

I assume you are to be working with those within an adult Center. What a wonderful thing. They need to be active and moving around. Not only tossed in a corner awaiting the next meal. You are merely giving direction for what your actions may become.
Just know that Seniors no longer have the dexterity others do. AND know how to handle a situation if Mrs. R begins to cry.
Seniors love having young folks around. If nothing else but to talk to them or read from a book. KNOW you can NEVER give too many compliments.
Do ask a nurse or one in charge if there are certain things you shouldn't discuss with a certain patient. Alzheimer's may present memories they have held back ...locked away for many years.
Do present back with us your notes. AND do not worry about your age being too young. It has been too long since the majority of seniors...kept in a nursing Home have not been around young people.

Prior to attempting to get one to speak out, ask the staff for direction. There may possibly be certain subjects that might break them down. Memories and of loss.

Guys, take a breath here and allow her to speak. Seniors are placed into nursing homes because the family can no longer care for them. Elderly adults come alive when young people are near. They can look back at their own youth and see and BE that person again. Their memories are all they have left remaining. To volunteer at a nursing home/Facility is the greatest gift you can give of yourself.
Try and get them to sing along with a song. To raise their arms up high and stretch. Choose a well known Nursery Rhyme. "Mary Had A Little Lamb" And have each person recite words their memory allows. What may appear simple for you can be Volcanic to a Senior whom has lost that part of their brain.
Your job is to open up that wall they have been hiding behind.
I applaud your efforts. Do NOT stop. Just take care in assisting their memory recall.
A loss of a spouse or even a pet can bring anyone to their knees.
Animals can work wonders with humans. One might be able to contact animal control to bring in a few rescues that can be petted or hugged.

If they're competent then I'm all for it

If they have the info and skills required then yes. E.g looking at teenage issues who better qualified than a teenager to put across those issues. Might then be up to an adult and teenager/s to come up with problem solving ideas. That's one scenario which I think it would work.

As a consultant, adviser, instructor, or guide. It might be the best idea in some situations. As an 'alpha', probably not.

Yes, if they have better skills and knowledge than any of the other members for whatever the committee is attempting to do.

Yes but it probably wouldnt work out. People wouldnt accept the power dynamic no matter how competent they are.

how old is the teenager ? 18 you are an adult and its experieance for anyone younger

What's the committee about?

If they're more qualified, sure.