Not only are today’s violent teens maturing into more violent young adults，but they are being succeeded by a new and large group of teenagers.

Not only are today’s violent teens maturing into more violent young adults，but they are being succeeded by a new and large group of teenagers.

not only…but(also)…指不仅……而且……；maturing into... adults 是现在分词短语作teens的定语。答案为：今天具有暴力倾向的青少年会成为更加凶残的年轻罪犯， 而且紧随他们的是一大批年龄更小的青少年。