I'm also Brazilian and I have the same perception as you about Maiden's shows here. Tour after tour the audience seems to be getting more and more inactive and less interested, which is frustrating. But, yes, your observation is as sad as it is true. We're all getting too old...
But, in our defense (or not so much), I have an account of the last show in São Paulo (Nicko's farewell): I got so excited and jumped so much that I seriously hurt my knee! At least in spirit, I still feel young. It's a shame that my body can't keep up...