I have delayed bone age does that mean my skull will increase in circumference once my bones have reached adult age?

Unlikely : Most will reach their adult skull volume by age 10, with minor remodeling thereafter.The sutures don't fuse between the skull plates until much later. After 10 your growth ins primarily in your long bones ( legs, arms) which in most males stops at 16-18. If you have some delay (bone age of 15) that would not be enough to make more head growth likely.