Im not sure if each state is different but in Minnesota you can be in 4-H one year past high school... or no older then 20 years of age. You also have to be in good standing with the community, no smoking, drinking, etc...There are many projects that you can be involved in from animals no non-livestock. there are a lot of youth that are from the city that will show a dog, cat, pets and then there are the projects ranging from flower gardening to bike safety, woodshop, sewing, animal science, health and safety, photography, and many, many more...
I was in 4-H my entire youth years with my favorite project being the horses!!!! the show in minnesota consists of pleasure (showmanship, western pleasure and horsemanship, English equ and pleasure, and trail class) and for gaming/speed events (barrels, poles, key hole, and jumping figure eight), there are also training levels that the youth do the training with a horse starting at a weanling age and work up to like 5-6 years of age before they test out.
4-h also has knowledge bowl teams and judging teams (for all livestock)... teams can earn their way to the national level. There many ways that youth can get into leadership positions as well. In Minnesota they also have animal interviews at the local and state level. It is a one-on-one verbal test of the knowledge (history, care, etc) of the type of animal. I was a medialist winner one year at the state for market goats...that was pretty exciting. Im not sure how other states work for the state fair but in Minnesota you show at the county level and are chosen to represent the county down at state. I was able to get a trip every year with non-livestock and livestock. and even got state horse show trips... the horse program was very competitive though but I worked my butt off.
I really encourage 4-H for families that can participate. I showed horse, beef, dairy, goats, sheep, rabbits, chickens, ducks, and my cat. I also did many non-livestock displays. One year I had over 30 some projects at the county fair. My summers consited of getting my animals fit and doing my displays. Now that I have graduated from 4-H I have gone back to my original county and donated time to the 4-H horse project.
I hope this helps out a bit...here is the national web site for the USA: Welcome to 4-H.org!