There are critical growth and development windows that in particular make motor learning easier.
If you get an active start under the age of 6, where you’re put in an unstructured but challenging and sensory stimulating environment, you’ll usually have an advantage over your peers in the next stage and all subsequent stages of development.
The most obvious window to start with basic (and usually bodyweight) resistance training is called the FUNdamentals stage (boys 6–9, girls 6–8). This isn’t necessarily the ‘best age’ to specifically start a routine for muscle but it is a critical opportunity to learn the movements that go hand-in-hand with building muscle later in life.
This is when agility, balance, coordination and speed are all really developed and balance/coordination are paramount to resistance training later in life. A person with good neural connections will have an easier time when they start to actually add load. Gymnastics and track and field are great options here for skill development (as are skiing/skating/swimming/cycling etc… for other reasons related to development).
The next growth window isn’t really considered an optimal period for muscle building either (you need puberty to hit for the hormones to be maximally engaged really) but takes the specificity of movement a step further (agility/balance/coordination and speed) because now you can probably start some deliberate resistance training with light loads.
Boys of 9–12 and girls of 8–11 are prepubescent but they soak up motor skills at this age, and can deal with more structured forms of movement. This is often called the ‘Learning to Train’ phase.
For example this is often the phase where we start more deliberately training ‘strength.’ We just don’t use heavy weights. We might teach kids how to use a dowel or light barbell, how to use a light kettlebell or how to use a light dumbbell but it might not be in a very formal manner. We use medicine balls, lots of bodyweight and focus on speed/technique of movement rather than weight. We’re not loading them to a point of failure or fatigue really, we’re just challenging them to move with tools now. This is a great age to learn tough training skills like squats, deadlifts, swings, olympic lifts, etc… We just don’t use loading as the marker for improvement and we’re careful with technique.
Things really start to take off in puberty though (the Training to Train phase) and post-puberty. This is when you can start to have a meaningful effect on muscle mass (13+ for boys and 12+ for girls typically) and when you can start adding load (providing technique is sound).
If you’ve done the early phases right, adding load should be fairly easy in this period and strength/muscle mass generally peak by about 27ish, not in everyone mind you but generally speaking.
It doesn’t often become clear until the Training to Compete Phase (16+ boys, 15+ girls) when structured training can really take off. Basically by this point you have the hormones in place and the emotional/intellectual maturity to really train effectively. You don’t always have that maturity in your early teens to train really hard, though certainly some kids do.
People with good movement foundations grow mass more easily and train more easily. Having all that foundation also helps with your perception of exercise. The longer your wait, the harder things seem/feel (people have different tolerances based on experience and genetics). The sooner your started, the easier things feel, even if you’re not yet training for muscle mass.
The earlier you can start training movement, the better, even if it’s unstructured but you shouldn’t really start loading movement until you’re over that age of 13 for boys and over the age of 12 for girls. Generally speaking. Chronological age and biological age can be different (some people are ready sooner than others or later than others based on how their bodies actually grow) but that’s the rough zone where I’d want my kid to start training for muscle if it’s possible.
The more muscle you have early in life the longer it sticks around.
All that being said, you can start at any time. Just because you’ve missed these windows of opportunity doesn’t mean it’s impossible, it just makes it harder in the long-term. Everyone benefits from a regular routine of strength or resistance training and will benefit from carrying a little extra muscle, especially later in life.