What is the average trainer level?

Looking at ur area please. That is where u play everyday. Hitting lvl 30 will make u feeling better, I guess.

I'm at half way through level 27. I live in a small rural town that only has 17 gyms total. I didn't start catching rare or high CP Pokemon until after level
20. But I noticed since the update, I'm seeing lots of eggxecutors, snorlax, jugglypuff, and slowbro's around here when I never did before.

I'm getting into level 30. That's pretty high where I live (Brasilia, Brazil). The avarage would fall into 25-27. But there are some "people" at level 33-35.

Of the people that I know, 25-27 seems about right. My peers are professionals in their late 30's to mid-40's. I'm at level 26. I live in Los Angeles.

I'm a 33 (Ey3con) in Santa Clarita, CA (Southern California) and my three roommates are 21, 24, and 27.

Hope that helps!

tl;dr - 1. Wiki readers and gym defenders will not give an accurate view of the "average" player. 2. Successful gym defenders probably only constitute less than the top 10% of players. 3. Catch every pidgey and weedle you see, and you'll get there soon enough. :)

Keep in mind, any poll performed on a wiki will result in a skewed sample. For the most part, people who take time to visit a wiki (especially those participating in the Q&A section) are people who are pretty serious about the game. They will therefore be higher level than the "average" player. Looking at the average level of players defending gyms is also skewed, because gyms favor the strong (and therefore higher-leveled). So even if the average level of all players were somewhere in the 15-20 range (my guess), the average gym defender would still be much higher than that.
As to your question, I think you are correct about the average gym defender being higher than 24. In my area (Seattle), the average defender is 26-27. I've been participating heavily in the gym scene since I was level 20, but I didn't hold a gym for more than 48 hours until I hit level 26.
Here's my theory (in case you care):
1. A good Vaporeon (the strongest easy-to-get defender) breaks 2000 CP around level 25. Having that '2' in the thousands place is a huge psychological deterrent to potential attackers.
2. Due to increased XP requirements, the leveling process slows down significantly after level 25, meaning you acquire more strong pokemon per level. (I'm currently 28 and have 5 Vileplume, 6 Victreebel, 2 Cloyster, 2 Exeggutor, 9 Vaporeon, 8 Jolteon, 10 Flareon, 4 Snorlax, and 2 Dragonite.) Having this wide array of attackers (of various CP) not only allows you to take out enemy gyms easier, it also gives more options for optimally training your own gyms up to "find something easier" levels. The only enemies strong enough to take down these gyms are high level themselves, which creates a cycle that keeps control of the gyms in the hands of the top 10% (guesstimate) of players.

level 40, soon to be level 43 when you count up my 44,000,000 xp.