"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. আপনি may not be smart enough at the moment, but that doesn't mean আপনি 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 আরো important than happiness? লেখা could be a side job if needs be.
"I believe in পরী and unicorns."
"How old are আপনি 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...