They divide kids by some arbitrary birthdate (Dec.31 or Aug.31 or April 30 or whatever).

But each of us grows and matures at our own pace.
So some 12s could have reached full height and can start muscle development, while some 15s are still waiting.

• You heal faster than your friends after cuts and bruises.
  Recovery and regeneration is quicker in kids.
• You can run around endlessly.
  As a kid you have boundless energy, a very good aerobic system - without training.
• You sweat water not oils.
  Your perspiration feels (and smells) like water, not saturated with body fat waste.
• You're still waiting for your fastest growth spurt.
  It's what scientists call peak height velocity (PHV).

 As you mature, some things about your body's strength and conditioning you can change, but some you can't, like...

• Genetic makeup. You'll turn out to be who you're meant to be. Some kids become super stars and some kids become teammates. But, hey, you don't know which you are yet so stick with baseball because you like the game.
• Muscle size. Muscles will grow, but not till 1-2 years after you've finished your biggest growth spurt (PHV, late teens and early adulthood).
• Muscle fibre type. Those with more fast-twitch muscle fibre are going to have quicker reflexes and be better fielders and hitters. The rest of us just have to learn to think smarter - be in the right fielding position, be ready soner for the 'fat' pitch.

• Muscle building. It won't do much but risk damage to ligaments (the 'hooks') and cartilage (the 'padding') at the joints between bones. Your cells require too high a training level for the amount of success you'd have in muscle building.
• Get bored. Too much repetitive, short-bust activity (like repeating the same drills, over and over) is not only boring it could result in imbalanced growth and poor skeletal development.
• And most of all what you shouldn't do...
  Worry about it. Instead...

• Learn skills. 95% of your neural network (your brain connections) is complete by age 5. That's years before your skeleton matures (during the growth spurt of early to mid teens) and way before your full muscle development.
• Weight train without weights. Let your brain learn the skill first and build muscle memory. You'll get stronger by learning to throw or hit or field efficiently - even though you don't build muscle mass. And you'll build limb speed - and flexibility.
• Have short practices. 75-90 minutes 3 times a week is better than a 3-4 hour weekend marathon. You want to increase the frequency of activity, but nothing that gets long enough to be boring.
• Cross-train. Play different sports, practice different skills. Think about how many star athletes played more than one sport as a kid - some still do!
• Have fun!