Unfortunately unless the band is as popular as say Nightwish or Children Of Bodom, they won't get the big venues that cater for all ages gigs.
They could have easily sold out a show booked for The Manning Bar which caters for all ages shows.
You haven't got that long to go now though, just a shame this year has been great for international metal bands. I take it you got handed one of the entry forms to go to Wacken next year at Bodom on Tuesday? You'll be 18 then if by chance you win.