I'm 21, and started with the original. To date I've played through I-VII, and IX. I loved the first six installments, and still hold VI as one of the greatest RPGs of all time. VII, however, felt like a cash grab to me. Mediocre gameplay, a story that made absolutely no sense, due to a shoddy translation and an astounding lack of character development (I was much more impressed with Wild Arms, which hit earlier that year if I remember correctly).
VIII was simply a chore to me. I sold it (I'm a person who NEVER sells his games, btw) after I cleared the first disc. I outright hated the characters, and the story seemed to bank on that "Burger King Reject" angst. I had better games to play, and it had to go.
IX felt like an apology for VII and VIII. They seemed to do so much right, after the disasters that were VII and VIII, with a great cast, a fairly good story, and an abilities system that didn't make your characters into virtual carbon copies. It cleared a lot of stumbling blocks, and shone through as a solid game in my eyes.
X, I never bought... I borrowed it from a friend. However, I have to say that I will NEVER be purchasing it. The game held the player's hand through the whole way, and spoonfed you a story that felt like a poor man's Grandia II. To this day, I STILL mock the whole "Yeah... yeah... yeah... Ihateyou... yeah..." dialogue.
I haven't touched an FF since... the series is dead in my eyes, and Tetsuya Nomura is the murderer.
If what I've read is true, and Nomura's kept his grubby hands off of 12, I MAY be inclined to check it out when the price drops.