I only last week tested TT at 376 which I consider low for my age (39). I still don't have much trouble gaining strength or size. You can see veins poppin out of my arms and shoulders, especially on the 60 lb curls. Many of the younger guys at gym think either I am on or have done roids, which I haven't. I attribute at least some of it to muscle memory.
I worked out like a madman from age 13 to 18 and was quite ripped back then. Am guessing that despite my low T that I must have a good number for Free T. I hope to find out soon as I look into this further.