It'll come down to a fair few factors but one of the biggest and probably the most uncontrollable is luck.
You can be blessed with all the natural talent in the world but if you're never spotted then you're chance of making it lessens each passing day. Of course you can maximise the probability of getting spotted by working hard at your game etc but you still need to be in the right place at the right time.
Take the recently retired Trescothick, when young he was tipped for big things but he then had 7 fallow years on the county scene, if it wasn't for him hitting a big ton in front of Fletcher at Glamorgan (iirc) then he would probably have missed the boat when it came to the international game. Yet, that one knock swung it in his favour when the national coach was looking for an opener for the one day team.
If we take luck out of the equation then it's going to come down to things like natural talent, application and mental attitude. Take Harmison for example, his has all the attributes (save for a slight flaw in his action) to be a great yet (for whatever reason) he's mentally weak. As mentioned earlier Ramprakash, a wonderful player but when younger he remained in his shell too much and often let the opposition get under his skin so his talent was wasted. Hick, a wonderful player on most pitches but a fragile ego - the list goes on and on.
Swinging this back around to what I think you want to hear, then at your age the best thing you can do is learn the basics, absorb knowledge from older, more experience players, practice hard, learn your own game inside out and never back away from the challenge (whatever it may be). Generally speaking if you're good enough you'll get a chance to shine, it's the years of hard work beforehand that divides those that are able to take it and those that let it slip from their grasp.
