When Should You Start Playing Ranked (How Not To Get Placed in Bronze)?
# of Normal Games
This is not the best of the indicators, but this is one of the better ones that you can use to decide whether or not you should start playing ranked when you reach level 30.
100-300 games played: I would say play more norms.
300+ games played: you are probably ready.
Tip: If you want to get placed in a rank higher than bronze, I highly consider waiting until you are very confident.
Mechanics
1v1 mechanics, team fight positioning, and etc.. can get you out of bronze easily or get placed in a rank higher than bronze.
So if you win 1v1's often during the laning phase of the game, you should consider playing ranked.
'Norms are Too Easy'
If you feel that you dominate every normal game, and you want more competitive play, I recommend ranked.
When You Stop Being a Little Baby
Ragers, toxic players, etc.. ruin games period. If you are one of those, please please please just. don't. play. ranked. Or even norms.
If you are going to cry about every single thing, just go play bots or create a custom and play by yourself.
In Closing
You can obviously play as soon as you reach 30, but following my 'guidelines' will help you rank up faster and get placed in a higher rank than most ppl.
Feel free to add more if you wish, and as always Have Fun and Good Luck :D