So, I have a question. What's the approximate level of jumping for different ages? For example, what age do most people start jumping crossrails, then verticals, etc.?

I started jumping over shit when I was about seven. I have not progressed beyond crossrails.

If you are talking about horses, I have a feeling you will not get a cut and dried answer. You are going to find breed makes a difference to many people in what age they start horses, as well as start them o/f. There is also the question of consistency. Jumping a relatively young horse over some relatively substantial jumps early on is different than training a horse regularly over fences. I think, for a broad answer to your question, you might consider looking at horse showing and seeing the median age of horses doing, say, the first year greens - this will give you an idea of what age horses are jumping what height.

It also depends on hunters vs. jumpers; the hunters probably start over bigger fences (3'6+) later than the jumpers because anyone who wants to do the young jumper division has to have either a 5 year old jumping 4', a 6 year old jumping 4'3, or a 7/8 year old jumping 4'6.

I'm talking about people.

Unless you are talking about young children, who may not, developmentally, have the strength, coordination, motor skills, etc. to get over a fence, I have never even considered age to be a factor in how high someone jumps. When an individual builds the skills, no matter what age, that's when they jump a certain height. If someone is stuck at xrails, it is likely to do with a lack of talent, not age.

Is anyone with a trainer who does have certain rules about stuff like that??

My ex-trainer wouldn't let kids under like 13 jump anything but xrails.

When I taught, it was always about ability, not age. If I had a 6 or 7 year old that was appropriately mounted, strong enough and steady enough, I would start them over fences. I also had teenagers and adults that I had on flatwork for what seemed like forever because they developed their skills slower. It really depends on the person.

same here.

I didn't know you taught, Tiffany!

Well I seem to have a grand answer. For poles I would say the ability to steer. For cross rails the ability to balance at either trot or canter (aka if the rider get left behind at the trot then it’s a no go). Then a small vertical, the ability to find a distance (though for some amateurs this does not matter He he). Then oxers when they master some pace control. After that confidence and of course the horse to progress upwards.
Ha ha if they want to do things to fast I say let them. For I know I was a bratty kid a while back when I was doing small jumper courses. One show I basically screamed my way into doing the children jumpers even though my trainer told me it is about 6 months before I was ready. Guess what? I did not get my stubborn butt around a children’s course for about oh what was that? 6 months. And I even learned that if I get told I should maybe wait, that I probably should though sometimes I do not.
So in other words no one knows.