I like to start horses jumping at 4yo, as long as their flat work is good and they are balanced at all gaits. I start jumping them over poles, xrails, and 2ft verticals. Once they are 5 I generally will stick around 2'6".
The key is to make sure they are balanced enough and trained enough to start jumping, and then not jumping them too hard or too often.
Gypsy & Scout <3Everybody is a genius. But if you judge a fish by its ability to climb a tree it will live its whole life believing that it is stupid. ~Albert Einstein