question about ages for the manga series

I'm a children's librarian, and the Naruto manga series is very popular. I know that some long running manga series will grow older with the audience. So a series may start being good for kids, but as it progresses the characters get older too and it later become more suitable for teenagers and adults (and less suitable for kids). So, that's my question. I know that at volume 28, there is a 2 1/2 year lapse, and Naruto is older and more mature. Would you say that volume 28 would be a good point to move the Naruto series to the Teen section of the library (for ages 14-18) and out of the children's section (for ages 0-13)? Does the story get more mature -- more violence, more nudity, more romance, more complicated storyline? Or does the level stay the same?

We will continue to have the manga series at the library, but I want to make sure it's in the right place so I don't have mad parents coming at me. Your input as Naruto experts is appreciated. Thank you!!