You can start pvp right away, usually matchmaking sets you up with people with around the same Elo score(and lvl I believe, but im not sure). Only thing you might wanna worry about are veteran players creating low lvl accounts to play against less experienced players.

Also, about runes, Tier 3 runes(which you can get at lvl 20) are the best runes at the game. Right now however there is a event that gives you special holiday runes(that are tier 2(lvl10 requirement) that give you about 90% of the stats of tier 3 but at a ip cost at less then half of tier 3. You might wanna buy these special runes even since lvl 10 isnt that hard to get to.