The strength (and cardio) gains you make as a child will stay. But if you stop training they will go away. But when you however start training again you will ha

The strength (and cardio) gains you make as a child will stay. But if you stop training they will go away. But when you however start training again you will have faster gains due to prior training. It all depends on how much you train during your life. While you are the strongest around 30 years old, you will keep most of it until 60 years old if you keep on training. While there are definitely advantages to have been training as a child, I don't think it will be significant when you are 40-50, it will more depend on how active you are during your life. And it's really never too late to start training.