I'm a 57 year old beginner, and I had a hell of a time getting the D down. I just - couldn't make it work for me. I just kept at it every time I picked up the guitar, until something in brain clicked and I could play it. I realized recently that now, whenver I put my idle left hand on the guitar neck, I end up fretting the open D major automatically. I assume I was doing that for a long time without realizing it.