Not sure about the laws in Germany, but as I said, BDSM itself is fine, however English law still doesn’t recognise consensual mutilation for sexual gratification as being distinct from hacking at someone with an axe.
As for the 18 thing, I think that’s more to do with a percieved “ok” ness involved as then they are technically an adult. Of the few fetish clubs I know, the age limit ranges from 21-25. Would help if you could say where you got the figure from, websites, magazines, tv.. etc, as AFAIK the law doesn’t actually specify an age, merely using the term “of legal age”, refering to the age of consent (currently 16)… of course, my knowledge is VERY limited on the subject and have only a glancing knowledge of the law surrounding BDSM and sexual practises.