Rust traits

struct Cat {
    name: &'static str,
    stripes: bool,
    age: u16
}

struct Dog {
    name: &'static str,
    barks: bool,
    age: u16
}

trait Talkative {
    fn say(&self) -> String;
}

trait Aged {
    fn age(&self) -> u16;
}

impl Talkative for Cat {
    fn say(&self) -> String {
        String::from("miau!")
    }
}

impl Talkative for Dog {
    fn say(&self) -> String {
        String::from("vau!")
    }
}

impl Aged for Cat {
    fn age(&self) -> u16 {
        self.age
    }
}

impl Aged for Dog {
    fn age(&self) -> u16 {
        self.age
    }
}

fn say<T: Talkative>(t:&T) -> () {
    println!("{}", t.say());
}

fn older<'a, T: Aged>(a0: &'a T, a1: &'a T) -> &'a T {
    if a0.age() > a1.age() {
        a0
    } else {
        a1
    }
}

fn main() {
    let animal0 = Cat { name: "Cirmi", stripes: false, age: 5 };
    let animal1 = Cat { name: "Mirci", stripes: true,  age: 15 };
    let o = older(&animal0, &animal1);
    println!("Older cat says: '{}'", o.say()); // Why compiles? 'o' IS AN Aged but IS NOT A Talkative ...
    // say(&o);     // this is better, it won't compile

    let animal0 = Dog { name: "Morzsa", barks: false, age: 6 };
    let animal1 = Dog { name: "Benji", barks: true,  age: 8 };
    let o = older(&animal0, &animal1);
    println!("Older dog says: '{}'", o.say()); // Why compiles? 'o' IS AN Aged but IS NOT A Talkative ...
    // say(&o);    // this is better, it won't compile
}