I have good extra curriculars, most of them with leadership positions. However, I have only been doing these activities for the past 18 months.
I pretty much didn’t do anything in 9th and 10th grade, and all my activities are from the 11th and soon to be 12th grade.
I know we list hours and weeks on common app.
Will this affect my chances? Is there anything I can do?
Thanks 
Yes. Be prepared to explain why you didn’t do any ECs for 9-10th grade.
Thanks.
Any other opinions?
Do you know the cliche “quality over quantity”? I think it applies here. There are people who join 10 clubs/ECs in 9th grade and stay with them all four years, yet never hold a leadership position or win awards, while other people join much later yet become captain/president and are ranked first in the state. I agree with ben69420, some colleges may wonder what you were doing 9-10th grade, but they may be willing to overlook it if you have amazing performance in 11-12th.
Okay, so the truth is my old school that I was in from 9-10th grade had no student-led clubs. At all.
To be honest, I didn’t even know student led clubs existed till I transferred to a new school in 11th. The one or two activities were teacher led, and there wasn’t any emphasis on co-curriculars.
I want to tell universities that I didn’t participate in many extra curriculars during the 9-10th grade not because of my own laziness, but simply because I wasn’t in that environment. When I came to a new school in 11th, and discovered these, I took opportunity of joining them.
How can I best convey that message? In a humble way of course, not to sort of blame my old school.
Thanks
Use your essays! That is where you get to shine and express yourself as a person and not a statistic! Consider writing about discovering involvement and leadership and how it has empowered you! There are so many great ways to convey your message through your essays. Did you discover your passion through your ECs? If so, what is it?