At what age should kids start "fitness training" for sports?

That depends on the interpretation of "Fitness".

This question came up in an online soccer coaching forum that I'm part of:

"My U13's will be playing on a bigger pitch this year, should I add extra fitness to our training sessions to get them ready for that?"

The overwhelming response from others in the group was something like: "Kids don't need fitness training, they'll get fit by playing."

Here's my issue with that: There are many dimensions of fitness, it's not just cardio endurance. While a 12/13 year old won't benefit much from maximum strength, anaerobic capacity or plyometrics (that is to say, natural development will happen faster) they will gain massively from muscular endurance, coordination, balance, control, body awareness, proprioception and mental strength.

We evolved through various movements and activities including walking, running, jumping, climbing, digging, carrying and more. The average kid today barely moves at all, relatively speaking. They get a ride to school, sit for 6 hours, no physical play at break time anymore, car ride home & play with friends online.

A 2 hour, twice a week training session that most youth athletes will partake in is performed on a weak foundation. Soccer involves a lot of fast movement, sudden stops, twisting, turning, jumping and explosive actions. If a kid has no core control, posterior chain development, body awareness etc (which would develop through the natural movements mentioned above), they will not be able to move correctly. For example, they will likely load their knees instead of their hips when making a quick, lateral movement. Over time this can cause some overuse injuries.

I know it might seem unlikely for 12/13 years olds to pick up injuries like that but I see it fairly often. The muscles, tendons and ligaments get little exposure to force and therefore don't have any reason to strengthen. Then they are overloaded with force during a training session or game.

As a coach/fitness trainer, I see it as very necessary to add the correct stimulation to developing bodies. That is the natural way we develop: Stimulation 》Adaptation.

It doesn't need to be a full session, it doesn't need to be separate from skill training, it doesn't need to be boring or forced. There are plenty of fun games, races, challenges etc to get them moving in ways that will create general physical fitness and ignite the spark of wanting to be fitter.

Not to mention, they will be expected to be involved in strength and conditioning training if they are still playing at 17/18. Especially if it's at a high level. So introducing it now will give them a head start.