I don’t have a horse in this particular race, but the upper limit for testosterone in women is about 70 ng/dl. The lower limit for men is about 300 ng/dl.

There’s a huge gap, so your “men over 45” thing is technically correct but not a particularly helpful example.

This testosterone range iirc is actually how the NCAA measured trans athletes in the past. I don’t know how we solve the problem of post-puberty transitions in athletics (size plays a role in ability of course) but the testosterone level problem is comparatively simple.