Junior BJJ belts are different from adult ones. How are they converted when a junior becomes an adult?
So let's say I'm 15 and have Orange belt. When I turn 17 should I start from White again or will I receive a Blue, Purple etc?
There is no official method since every school has different rules. However, it is quite standard to convert all the belts from the yellow up to a blue belt. This is because there is normally no sense in letting these former under-16 people fight against white belts. On the other side, if they are much better than a blue belt they will prove it quickly during competitions and they will then be rewarded with the purple belt.
As @Annoys answered, it varies between gyms.
My oldest son is a purple belt aged 19; my younger two, aged 13 and 12, are grey and white belts. When my younger two turn 16, they will receive blue belts in the gym they go to, based on years of experience, so a yellow belt and a green belt at the age of 15 years and 11 months, may both graduate to blue belts.
My oldest son started at 15 and was still a white belt when he turned 16; it was just a matter of getting his stripes before graduating to a blue belt.
In some other gyms they will only give belt promotions if the person passes a grading; no automatic promotion.
In other gyms, children who have been training long enough will graduate to purple belts at the age of 16.
From the IBJJF