If a 17 year old is about to turn 18, do they still DNI?
TLDR; yes.
up until the day you turn 18 you are legally considered a minor. once you are legally an adult, you no longer have to abide by the DNI rule.
i know for a fact that my DNI list won't stop everyone— i was a teenager too, not too long ago. not to mention i was on the internet at a VERY young age, exposing myself to adult content and fiction that most definitely fucked my brain a little bit. i encourage you, and anyone else wondering the same question, to just be patient as much as possible. everything will still be here.
i know that DNI criteria on blogs or accounts can only go so far. unfortunately, i'm well aware that the only way to keep an adult space fully adult is through ID verification or keeping the account private. on tumblr, you can't really have that privacy, so having an honor system is the best we can all do.
but for peace of mind; the greatest attempt anyone can make to keep adult and minor spaces separated is to have a MDNI. i would prefer if you waited until 18+. otherwise, interactions from ageless or minor blogs get blocked. this isn't just for my own safety but for yours as well. like i said, i was a teenager not even 5 years ago. i get it, i really do. but i don't feel comfortable as a now adult having minors interact with mature content of mine, and i would never interact with a minor's content from an adult-centric space, either.
i hope this answers your question, and i hope it makes sense.