Do things like slight cow hockedness improve with age? I have never had a yearling before, so all new to me. Also is it something that careful trimming can help correct?
If the closeness of hocks is due to lack of muscle and stength then this should improve with maturity, correct feeding and suitable natural exercise.
If it is due to the fact that either the second thigh goes into -- or the cannon bones come out of -- the hock at an incorrect (side) angle then no, nothing is ever going to improve that kind of conformation as it is structural rather than developmental.
When I bought my mare as a 3yo she had dodgy conformation. I didn't have much money at the time. I bought her with the sole purpose to hack, couldn't see her doing anything else. She was a bit back at the knee and slightly cow hocked but she was very calm and I could see she would make a great hack. She is now rising six and has totally changed!!! She has strengthened up and is no longer back at the knee at all and you would never know she was cow hocked as a youngster. My farrier hasn't tried to alter her with trimming and shoeing, she has just muscled up, widened and matured. She has been shown, jumps BN, hunts, events, dressage and she is still the best hack anyone could wish for!!!