Let's get one thing clear - at the time of writing, I am 17, and I've been reading these books since around the time of the fourth book, The Assassins of Rome, which came out in late 2002. Ever since, I've been hooked and determined to read through to the end despite apparently growing out of the 'recommended' reading ages given by Amazon and such places.
I have not been disappointed. Though the books were published over a span of eight years whilst the story is spread over two, the characters and the themes definitely mature throughout the series, and the final book is not excluded from this. In fact, some of the darkest and cleverest scenes yet in the series definitely provide a route for anyone to be enthralled in the words on the page. I found myself finishing the book at roughly 3am last night.
Fans of the series will be desperate for various loose ends to be tied up, and they will not disappointed by the results. Imagine a situation progressing in the book - as you read, you slowly imagine three different ways it could unfold, only to see all of them have a chance to unfold and then being shocked as a fourth solution bounds onto the stage. The various twists and turns of this book keep going right up until the last page, surprising you as they happen but ultimately rolling together and making sense at the end.
If you are new to the Roman Mysteries series, I would strongly recommend you start at Book 1 (Where else?) and read onwards. Reading a random mismatch of the books in no particular order will only spoil your enjoyment of them, potentially giving away spoilers that you haven't read in previous books yet and just generally confusing you - even I found myself flicking back to a previous book to reaffirm what had happened at one point.
But the summer is approaching, so I recommend you order all 17 now and make your way through them over the coming months. The length of each book is roughly similar and easily readable (No gargantuan Harry Potter beasts here), whether you've been reading for many years, or you're just approaching the world of reading for the first time.