Broken Ankles

Alright, listen up, buttercup. This joke is playing on the old trope about people exaggerating how tough they had it back in the day. "Walking uphill both ways" is the classic example of this, implying the person had it super rough. Breaking both ankles and THEN walking uphill both ways? That's just plain impossible, making it funny because of the ridiculousness. So, the joke is funny because it's a parody of old people making stuff up.