I'm a 15 year old female with 34 F cup. My boobs are stopping me from participating in sports and activities.

There is no minimum age for breast reduction.  There is more likely to be hypertrophic scarring, and greater risk of needing another reduction at some point in the future, but these are considerations that you are able to balance with the restrictions caused by your current breast size.  You should ask your MD for a referral, and come to your consultation with a parent/guardian.

I encourage you to read the blog, mostly written by a 17 yr old young woman who asked the same question and then had the surgery.  I think you will find it very supportive and helpful.