I believe that Bnos Sarah is 10 years old, but I’m not sure. (I know someone who went 2006-7 and I don’t think that she was there the first or second year but I could be wrong.)
I go to a seminary that has been around for a very, very long time (seminarily speaking, at least- I know a girl who was the third generation there in her family) and it’s a fantastic place, but I also know of girls in much newer sems who have had a wonderful year. Machon Raaya, for instance- I’m not even sure if it’s five years old but it’s now really put itself on the map and all my friends there love it. And then there are older sems that have sketchy things happen (like Pninim- whatever you may think about that whole thing, it was still an established sem that went through a very rough period). You really just have to know what you’re getting into. Perhaps a sem that’s not in its first or second year would be better for you, but an arbitrary cutoff like “10 years” is probably unnecessary.