"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. You may not be smart enough at the moment, but that doesn't mean you 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 more important than happiness? Writing could be a side job if needs be.
"I believe in fairies and unicorns."
"How old are you 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...