I'm not good with coffee either

The Japanese have weird idea of what being an adult is like, or at least the adults think the youth do.

There's actually some science behind it — as you grow and age, so do your taste buds. Kids pretty much only respond with favor to "sweet", "salty", and "umami"; as you mature, you gain first an appreciation for "sour" and then "bitter".

I think the evolutionary biology theory is that it's a safety measure; sour and bitter flavors are more likely to belong to substances that can be harmful in large amounts, so they're reserved for when you have greater self-control.