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.

One of the benefits of lifting heavy things repeatedly over time is increased bone density.

Do we know if devices like Emsculpt work to improve bone density? I don't know how the cell-signalling cascade works for putting more calcium-mineral matrix in bones so maybe muscle contraction repetition initiates the cascade, or maybe it's insufficient by itself?

Full range of movement training also works to shift lymph back toward the left subclavian vein where it joins venous circulation.[1]

The lymphatic system is the primary cellular waste product transport system, and an immune organ in it's own right.

From what I read about the place this is a significantly understated feature of exercise and plays a key role in the idea that bed rest is massively counter productive in convalescences and post surgery recovery / outcomes.

1 https://en.wikipedia.org/wiki/Subclavian_vein

TENS units have been around for decades. Every once in a while someone tries to turn it into an exercise machine, but this is mostly marketing, not anything new.

I use my motor neurons to electrically stimulate muscle contraction and it doesn't cost me anything. I mean come on... the wires are already hooked up right in to your brain...