"I want to be a scientist when I leave school."
"You're not smart enough. Look at those grades."
Knowledge is acquired over time, years and hard work. u may not be smart enough at the moment, but that doesn't mean u can't make your dream come true.
"I'd like to make a living as a writer."
"Don't bother. It's not good pay."
Is money meer important than happiness? Writing could be a side job if needs be.
"I believe in fairies and unicorns."
"How old are u really!?"
If something is real to you, then it exists. What about Jesus?
"You're 20 and you're still a virgin!?"
There is no set time for anything...