My first introduction to Table Top RPGing came when I was around 13 to 14. A friend gave me some books and said, "Read That" without much explanation. That was actually the first basic set of Dungeons and Dragons rules from TSR. I thought it was a pretty simple concept to grasp save for the stats, but after awhile I got the hang of it.
I would suggest that you start with 3.5 D&D (have to admit to being a big fan so cant say I am not totally impartial)
It's the latest in the line of the most extensively researched rpg in the world. The D20 system it uses was designed especially for its immediate predecessors and it has built on it. More work has been done on it then any other table top rpg and it shows in the way it plays out. It's 3-core text books are remarkably easy to follow with a myriad of examples to show how things should be played particularly. They are also by far the easiest books to get your hands on and have the greatest number of GM's and Players. The last RPG society I was a member of had 60+ members, some played Star Wars, others Trigun, a few Bloodlines, etc... More then half however played D&D and we had over 6 campaigns gong on at the some time.
It also gives a very solid foundation from which to move on. The players handbook is used by a host of to other rpg games, and is without doubt the most useful D20 book you can buy. Furthermore the D20 system is the most widely used and once you understand D&D you will find it easy to pick up a myriad of other games such as Star Wars RPG.
Edited: Roehug on 16th Mar, 2006 - 5:32pm
I too started with redbox D&D, at about age 14, though I couldn't grasp it and it sat in my closet for a couple of years until I decided to get it back out and give it another try, once I had a friend who was interested in it - we struggled through the stats and prerequisites but for the most part, once I really read through it, it was all fairly simple.
Not so much AD&D, though it opened up a lot more options. I personally detest the new 3.x and all and don't like the D20 system in any incarnation - I feel its far too detailed and overcomplicated (though its basic system is simple enough) ; I recommend Warhammer Fantasy Roleplay, Call of Cthulhu, Feng Shui or Fudge, or even Wushu.
I think the difficulty comes in based on who you play with. Some guys like to know the exact height, depth and width of everything. Others don't care about the technicalities and just try to enjoy the story more.