"We have groups that do that, but I can't rap with the mentality of an 18 year old when I'm in my 30's"

This quote by Ice T speaks to the idea that as we age, our perspectives and outlooks on life change. He is stating that although there are groups of people who may still think and act like they are 18 years of ages, he himself can not do the very same. He remains in his 30s and has a different mentality than he did when he was 18. He has most likely experienced more life events and has a various outlook on life than he did when he was younger. He is likely more fully grown and has a much better understanding of the world around him. He is saying that although he can value the mindset of an 18 years of age, he can not relate to it in the same method. He has grown and changed and his perspective has moved. He is saying that although he can understand the mentality of an 18 years of age, he can not rap with it because he remains in his 30s.