When we talk about soccer skills, we mean the basic technical skills of ball receiving, ball control, passing, shooting, and 1v1 moves. These skills are best and easiest developed at an early age. It has been stated that it requires 4,000 ball touches per week to develop excellent fundamental soccer skills. So what is the right age to start? The answer is as early as possible. When kids learn to walk, get them started with a foam soccer ball and have them kick it around as they walk. Then play games with them where you throw/pass them the ball and they kick it back to you. Keep this going as they develop and get older. Tapping a ball between feet while watching TV can generate tremendous ball control and feeling for the ball.
As far as organized soccer in clubs goes, it is difficult to achieve 4,000 touches on the ball, hence the “home work” mentioned above and playing pick up soccer with friends is recommended. But soccer coaches can help. When you plan your practices, make sure that players do not stand in line waiting for their turn in the drill. Instead, break each drill into small groups where everyone touches the ball at least 10 times per minute. Some simple things can help. Ask your players to “dribble” the ball from the parking lot to the field (and back). When a particular drill is done, ask players to walk the ball back using both feet, instead of picking it up or have the coach kick it back. Every touch helps.
The later in life kids start maximizing touches the harder it will be to develop skills. A young athlete with mediocre skills trying out for a High School or College team will be at a disadvantage against someone who has advanced their skill through repetition. At that stage there is little the coach, or the player, can do to make significant improvements.