As far as I am aware, you don’t get paid if you are under the age of 18, regardless of your read numbers or gems. As it states in the payment info on the portal, that you must be 18 or over.
As has been mentioned, you need to receive (within 60 days)…
Either 500,000 reads on your stories or 100,000 gem chapter reads and 300 gem choices made. So you don’t just get paid for adding gems. That just shortens the amount of reads you need in that short time frame.
Given that you’re still 15, I wouldn’t be worrying too much about the payment programme due to the age requirement.