The police probably don't care since
- you're too young to be ticketed
- at your age, you're likely to give them lip and ride on anyways, and there's nothing they could reasonably do about that
But when something happens, you might just be old enough to be liable (over here, at your age, that's a case-by-case decision of the judge), and having violated the laws (whether you know them or not) is not really likely to reduce that liability (and may, depending on circumstances, free your insurance, should you have one, from having to pay up).
Basically, all over the world, bicycles are legally vehicles. In Germany, the critical age boundaries are 8 and 11 years*, so you would be over both and would have to ride your bike in traffic like a vehicle. And, similar to other vehicles, your bike would have to fulfil certain requirements. Usually, that's lights, brakes and a load of (usually useless) reflectors. As for the helmet, you'll also have to check your local laws.
*until the age of 8 (IIRC, for both boundaries, as I am far above either) you are required to ride on sidewalks and, in general, obey pedestrian laws. The legal workaround for that is that kid's bikes (which are not further defined) are classified as "toys" instead of "vehicles"; between 8 and 11, you are permitted to ride on sidewalks. Liability for damages also begins somewhere around that age - like I said, if push comes to shove it's usually a case-by-case decision, hinged on whether it is plausible that the kid in question could have had an idea of the consequences of their actions.