It's strange, I haven't met lvl 30 while leveling before I was over 20 myself and what was with close to 70% win ratio. At that point having a lvl 30 on a team seemed like a big disadvantage. You have one player who is a bit stronger and then you have one player who is beyond useless. Maybe if they duo a lane with the low level being a support it maybe can work, but send him to any solo lane and it's a gg. Even on support with no talent points and no runes he is glorified gold generator for enemy team unless he starts with cloth armor and pots.
I still remember a match where we had two lvl 30 players + two low levels vs enemy team with one lvl 30 player and one low level ... I didn't like our chances and we did indeed lost the match.
So I guess it's troublesome thing to balance. Send low lvl + 30 against lvl 20+ players and it's a gg. Send low lvl + 30 against <20 lvl players and it's gg the other way around.