Hello Rachael
As Julian has said you would be in the younger age range, Most I have been on (I have done 6 tours) has been 55-70 age with people either side of that age range as well. But they are great fun and the young and the old all mix in together, I am 52 and am happy to hang out with the younger ones or the older ones, I love meeting new people and seeing great countries that's what makes the holiday for me.
Me personally I would just BOOK IT, and not bother with a discovery weekend, but that is up to you I know lots of people have enjoyed these weekends and going on one has inspired them to book a HOLIDAY.
The group sizes on a tour do vary, I have been away with 15-20 people but then again have had 35-40 people.
Cindy