For MN it goes by grade up to age 19 or 20... K-13 (13 being one year past high school)
Here we are able to lease animals with the 4-Her needing to provide care for the animal- by care the animal doesnt have to live on your property but you have to make arrangments were the animal is properly taken care of (like boarding). In the county I was in we had to be part of a 4-H club and then we could take part in the horse project. It is my understanding that every state does something a bit different.
In MN we needed to be enrolled by April (if I remember right) have you horse owned or leased by May 15th with the paperwork in the office, and then from there you have to watch and follow other guidelines and dates. Besides horses though you can show other livestock and do non-livestock projects like sewing, needlearts, posters on various things, photos, flowers, etc.... Everything is judged on a county level and from there you may be chosen to go to state and compete. The MN 4-H show consists of showmanship, western pleasure and horsemanship, english eq and pleasure, trail, barrels, poles, key, and jumping figure eight. From this show we compete to be top in our division and go to the state show. 4-H members must have completed 6th grade in order to qualify.
With our horse project we had a riding clinic each spring which covered basics to little more advanced, you could also get involved with knowledge bowls, judging, speech, I personally started holding weekly free lessons for younger memebers.
Here is a link that is from the national 4-H along with the Florida website. The one thing I can suggest is calling in and finding out who your area contact is and start getting involved. But remember one thing... You will only get out of 4-H what you put in!!!! Join 4-H Florida 4-H Youth Development Program - Institute of Food and Agricultural Sciences - University of Florida