Hi
Bad breath from mouth can be of local origin or can be of extra oral origin, mainly your gut. But at your age, I doubt any serious extra oral condition can be a cause. Please get your teeth checked for calculus and decay. If the calculus is present, no matter how frequently you brush your teeth, bad smell will not go.
Please get the scaling done from your dentist. Also check for any cavities .
If these local causes are not present, then I would suggest you see your internal medicine physician to rule out any extra oral origin.
Next Steps
Get your mouth checked for calculus and dental cavities. Get them treated first.