When talking with people who we meet on online, even though they are younger than us, we should speak in polite speech to each other. That's a manner in Korea. That's Netiquette. As we build a relationship with them for a long time, and then only we consider them as friends, not acquaintance. And then we could speak in 반말. But, in spite of a long relationship, sometimes, it would become no friendship from unknown cause. Therefore, we seldom, if ever, speak 반말. An acquaintance is only an acquaintance, not a friend. Korean people do not like "superficial friendship", as far as I know.