Across the armed forces, the minimum age allowed for enlistment is 17 (with parental consent) and 18 (without parental consent). The maximum age for enlistment for someone who has never served in the military before varies by branch; for the Army it's 35, for the Navy it's 34, for the Air Force it's 39 and for the Marines it's 28.
The year of the horse means there is leadership associated with this sign this year, so your authority may increase, according to Ng. However, the impulsive behaviour of the horse can lead to recklessness, so stay away from gambling and speculations.
A green horse (or rider, for that matter) is a horse that has little or no training. Usually a horse is referred to as green after it's saddle broke, otherwise it's considered unbroke, halter broke, etc. When it comes to showing, a green horse is often a horse that has had less than 1 to 2 years of showing experience. Other times it's a horse with less than 2 years of under saddle training.