I'm not sure how big your group is but my sister and I went as a pair and so did some room mates we had at our host families place. Now 3 would probably be reasonable, 4 might be safe but I would say anything above that would be pushing it for sure. The host family we stayed at held 5 of us at one time so I know its possible but it would depend on how many people the host family can hold and how...
I'm not sure there is an age restriction, but for most of the programs you have to get a background check and in my state you must be 18 to acquire one. I would recommend the program only to people 18+ anyways.
You can see a lot of reviews here: https://www.volunteerhq.org/volunteer-abroad-reviews/ There are 12 different projects in 50+ locations, so there are lots of options! :)