It may very well not be-- I've never been to such a group, but from what I've heard from others who have done group therapy is that other than the common thread -- in this case: low self-esteem -- the groups are usually quite varied. You might be a bit on the younger side, but not necessarily the only under-25 in a room full of over 50s.
But, and I mean this at face value, and not in any derisory manner whatsoever, what good does worrying about it do? The group has already been formed, and you are committed to going to at least the first session, if not all six. If there are people who are older than you there, then there could be the opportunity to learn from their experiences, or at least put a human face to the fact that low self-esteem is incredibly common. Your anxiety is messing with you.
Try a little mindfulness exercise: looking objectively, are there any stressors in the room with you right now? I'm guessing that you're at home, someplace comfortable, in front of a computer-- probably a laptop. You are in a very comfortable place, where your physical needs have been met, and there are no physical or psychological threats
at this exact moment, yes? I know from personal experience that anxiety is not rational, but you can use rationality to help limit it when it gets out of control-- the future has not yet happened, therefore there is no reason to get anxious over it. Plan for it, sure, but building expectation for future events either causes stress and anxiety now or huge disappointment later. Things very rarely turn out exactly how we expect they will; for people like us who tend to have a negative outlook on future events, IME it is
never as bad as I expect, and if things go badly it is in an entirely unseen way; for those with rosy outlooks (I've been assured that they exist

) or those who are planning a large, positive event, the expectation for how good it will be usually eclipses the actual outcome, leading very often to disappointment.
The more that you can focus on the present, the less such worry and expectation will be able to cause you pain. It's a learned skill, not unlike CBT in many ways, and in the same way it takes time and practise to become adept. If you look around online, there are tons of resources - some better than others, as always - about mindfulness training. I think that you would benefit greatly from it; I know that I have!
Best of luck with your session, by the way! Do let us all know how it goes. I can't speak for anyone else, but I know that I'm interested in what such a group might be like.