Alright, I'm going to intervene and tell you multiple reasons why playing vs lvl 30's when you're only level 20 or so is actually a good thing.
First, and most importantly, it offers you a chance to play against players that are likely better than you. Being at a disadvantage allows you to learn and step up your own skill.
In the words of another poster,
Originally Posted by JcGNeon
I started queing with level 30s as soon as I hit 20. It was nothing that playing better couldn't overcome. Of course I bought runes when I hit 20. Facing level 30s is quite the opportunity for learning experiences, and if you win, then +1 ego.
Secondly, it offers you a chance to see how champions should ideally be played. Instead of raging and getting angry about getting rolled, stop and think about how the runes, masteries, item build, and playstyle of the champion work together to make them effective.
Also, excessive amounts of bot games should not be played to get to level 30. For example, when I started ranked after I hit 30 last season, there was a player on the enemy team (he played graves, i recall) who was absolutely horrible. I checked his stats after the game, and he had won 0 normal games.
Bots just don't give you the experience that a pvp game will.