Hi Jimmy,
That's a tough question to answer. I think you're never truely "ready" as such... you just decide that you are comfortable with bowling the bigger events and taking on people who are perhaps more established with their game than you are, and want to challenge yourself. Often, people that average 220 in league will go awful in major tournaments, because they either don't have the experience in that environment, or they don't have the know-how to score well on the tougher conditions. On the other side of that, some people that average 200 or so can go well in major tournaments, perhaps because of experience in that environment over a number of years, or because they can play the tougher shots.
I don't know you or your game or where you're at in bowling, but I suggest that if you're only starting out, or have only been bowling a short time and perhaps have a mid-range average, work on your game really hard. Practice sparing a lot! Because that's something you'll need. I recommend getting a coach if you don't have one already, and talk to them about what your aims are and what you want to achieve in this sport. Work on setting goals, perhaps if your on 150 average, aim for 165 by the end of next year. If you achieve that, or come close, aim for maybe 180 by the end of the next year, and so on. When you get consistantly above 175-180, you may want to seriously consider regularly bowling in local events. When you get to 195-200 consistantly, maybe step up another notch. But it's really about when you feel you're ready and reasonably confident.
If you are under 18 I suggest trying out for President's Shield next time. You gain a heap out of the experience, even if you don't make it. If you have a few years left in juniors, it gives you experience for later years. If you're in the youth age group, try out for youth. There is nothing embarrassing about gaining experience in this sport, and I think some bowlers forget that at times. If it's your first attempt and you come last, it's a valuable experience bowling at a higher level, and you'll be better off for it in the future. Forget about the result, and focus on where you can improve for future events.
Anyway I hope that helped somehow.
Tim.