It ultimately depends on the player. There isn't necessarily an abundance of experience or knowledge needed to play League. The items are fairly straight forward, and only a fraction of the rooster sees frequent plays.
If you commit yourself to mastering one role, then you can probably compete against other level 30 players.
Two things to note though:
- Level 30 players will often have better champions and more masteries and runes than you. The innate advantage is quite large, and you won't be able to catch up until level 20. Unless you intent on playing support or jungle, early game will be really tough.
-The worst level 30 player is probably better than the average 1- 20 player. Many people at your level are still learning the game, while level 30s will be adept in many different situations just due to the sheer amount of games they have played. Expect a sudden increase in difficulty.
Either way, there is no way to find out unless you try. You don't lose anything in normal matches, so just queue with your friends and see how it goes.