"Haa? If they don't believe it, show them exactly what you can do! Unless they're idiot who wanna deny fucking reality." The concept of a hero being laughed at because he's 'too young' to be a hero, when he's obviously capable and acting as one, is such completely lunacy to Bakugo's mind. In his world, people wanted to be heroes at a young age, there were schools and programs to produce heroes, your goddamn Quirk was registered with the government when it manifested! Peter's world is so different despite being a world with superheroes in it...

"How many people in your world have superpowers? 80% of my world's population has a Quirk. A lot of them fucking suck though." You're not gonna become a hero if your Quirk's nothing more than an extra thumb on your shoulder, for example.