My eyes were white when i was younger but as i turnd teen, it started to get very brown. my doctor says as long i see well its fine?

That is right: If concerned get a second option ,by an ophthalmologist to make sure it's nothing out of the norm .

Common, no treatment: The white part of the eye is called the sclera. In the first 20 years of life, brown or gray spots can appear there. This is called scleral melanocytosis. Melanin is the skin pigment involved. Clumps of melanin cause this and also cause freckles. It is most common in sclerae of people of Asian and African descent. It is pretty common, like freckles, and is not harmful. It is rarely treated.

See ophthalmologist: Eye color doesn't change naturally. If you see a change of color, it needs to be checked by ophthalmologist and not PCP. Ask for referral to the specialist.