I don't know about CA or USEF rules, but I couldn't show our AQHA stallions until I was 18.
I use my oldest stud for lessons, mostly my intermediate riders as he has all the buttons, he is also ridden on a regular basis by my 6 yr old daughter. At home is much different than in a show pen where there are many more distractions & other stallions in the arena to be concerned about.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.