I'm 37 years old my bra size is 36h and I want to be 36b. Will insurance cover my breast reduction?

Once you see a plastic surgery, she/he will request an approval for surgery from OHIP.  If you wear an H-cup bras then most likely OHIP will approve the surgery.  

Breast reduction is a great operation to help relieve back and neck pain. In Ontario this is usually an insured benefit.  You will likely be covered by OHIP.  You will need a referral to a Plastic Surgeon close to you. They will apply to OHIP on your behalf.