I started lifting weights at 37 after 10 years of being very sedentary, and within a few months people were mentioning that I had put on muscle. My impression was that sarcopenia doesn't start until the 60's and even then there are benefits to resistance training.

Reduced physical activity in old age will accelerate sarcopenia. If you're hospitalized, especially for long period of time, that will also accelerate muscle loss.

Those are called beginner gains, especially if you've never worked out before.

The beginner gains are great, but the plateau hits most of us.

In my case it was regaining strength from taking weight-lifting semi-seriously in my 20's. It was actually amazing at how fast muscle can come back even after a decade. But I continued lifting and am stronger now at 38 than I ever have been in my life. What I am saying is that mid-30's is hardly when sarcopenia sets in. I'm sure it's individualized, but I don't seem to have a harder time gaining muscle now than I did back then; for me it's always been joint/tendon issues that have been limiting anyway.

> It gets extraordinarily difficult to add new muscle as you enter your mid-30s

Only because it requires you to create a major new habit after 30 years on earth of not doing it, not because it's difficult physically. If you are putting in the time and especially the effort, it should be easy.

That said, the common mistake in the gym is when people just kinda do reps at some easy weight and then stop short with a bunch more reps left in reserve. Never really pushing themself. I guarantee that describes anyone finding it extraordinary difficult to build muscle.