It was only a year or two ago that I completed my first ever series, and I'm 24. I don't like marathoning series of books in a row as I feel it gets boring, so I used to read books in between and then didn't go back to the series until I'd completely forgotten what had happened in the previous book. Stupid I know. I would then reread the first book and then the process would begin again. Anyways, here are the series that I am proud to say that I have completed:

The Hunger Games - Suzanne Colins
I believe this was the first ever series that I actually completed. I absolutely adored The Hunger Games when it first came out and was up in arms about it like the rest of the world - for me, it was the first dystopian book that I had ever read and it is the book that started me properly reading again so it obviously means something to me in that respect. I did feel however that the series slowly started going downhill from there, (don't hate me) from Catching Fire to Mockingjay which was the worst of the series. Mockingjay just ruined the whole series for me; it was predictable, boring and the characters were not who they were. Obviously things have changed and you do expect the characters to have changed, but not to the extent that Katniss changed. She had none of her traits of the first and second book and was just another follower which really disappointed me.
The Infernal Devices - Cassandra Clare
This is one of my all time favourite series - fact. In my eyes, it's the most perfect young adult series I have ever had the pleasure of reading. It's got a kickass female lead, a love triangle to end all love triangles, it's action packed and absolutely hilarious. Now, with regards to love triangles, I normally can't stand them. This one is completely different however and the book didn't need to revolve around it to make an amazing story; it was just a subplot - it's what young adult is missing in my opinion. Anyways, if I'm going to recommend any YA novel it's this one, just read it!
Delirium - Lauren Oliver
This was the first series of which I preferred the second book over the first book; the first book was a little too wordy for me and I felt as though it could have been more concise and the second book seemed to read my mind and improve on everything that I had opinions on. I actually enjoyed the love triangle in this book, despite my aforementioned comments on them, due to the fact that this series was about the idea that it was thought of as an illness to fall in love so it was kind of inevitable that a love triangle would emerge. The last book was brilliant but just ended. Fullstop. I was extremely angry for reasons I don't want to disclose because of the spoilers, but I had to call my friend at the time of finishing just to rant about the ridiculously awful ending.
The Maze Runner - James Dashner
This is the latest series that I have finally gotten around to reading so it's pretty fresh in my mind. This is one series that I can't make up my mind as to whether or not I like it; it was really stupid and really amazing at the same time. Although it has fantastical dystopian themes to it, some of the ideas were really ridiculously outlandish and annoying. The last book was my favourite of them all despite the fact that I felt more questions were raised than were answered in this book and in the whole of the series and it ended really strangely because of this.
Percy Jackson and the Olympians - Rick Riordan
Percy Jackson, sigh, this series was fantastic from start to finish. It was packed with action, adventure and learning. As much as I enjoyed the story through the series, I learnt so much whilst reading it on the different Greek Gods; their stories, their names, what they did, etc. It was very predictable but it was extremely fun to read and if a 24 year old can read this series and get enjoyment out of it on so many levels, imagine what it would do to its intended age range (not that you can put an age on a book).