You're going to get a variety of ages at parties, from 18-70+, but mostly in the 30-50 age bracket in my experience. Not many off-putting sights, but occasionally yes...but I always try and focus on the girls and their reactions. Great fun, you should try it, it wouldn't be a lot to lose if you didn't, but a cheaper way of getting your group sex kicks than booking a girl independently.
Finding a good quality party in Birmingham might be tricky. From what I've seen they look a bit BBW/MILFy for my liking. There is Hush Parties in Nottingham which have some younger girls.
Or maybe you could find another Midlands punter in his 20s willing to do an MMF booking with you, the regional section would be best for this kind of request.