Some excerpts from this Yahoo article:
You must be comfortable skiing fast and in-control. Some downhill skiers enjoy going down the hill at a leisurely pace and balk at the
thought of going fast. If you're not comfortable speeding down a hill
while maintaining your control, ski jumping might not be for you.
Speed isn't much of an issue when you're first starting out, but when
you progress to the higher ramps (40-meter and up), you will need to
feel comfortable skiing at a fast pace-sometimes up to 30 miles per
hour-to work up enough momentum.
Ski jump lessons are for all ages. Most ski jump areas give lessons for ages 5 and
up. In my adult-only class, there were about
six other beginning ski jumpers, including a 62 year old lady who was
taking ski jump classes with her 19 year old grandson!
You will start out on a tiny ramp. Beginning ski jumpers don't climb to the
top of that big, scary ramp and hurl themselves down, catapulting
themselves into midair-they work their way up to it. I started out on
a 10-meter jump, which is very common ramp height for first-time ski
jumpers. Don't let the height scare you away! The 10-meter measurement
refers to the distance a skier can get from the jump, not its height.
The 10-meter jump is only a few inches high-you're barely off the
ground. As your skill level improves, the instructor will move you to
steeper and higher ramps. The instructor will never move you to a hill
that's too dangerous for your ability.
A 2002 article shares some information about how Mitch Kaplan learned to ski jump at Olympic Park in Park City, Utah:
They call the program Public Ski Jumping, and anyone who can downhill
ski at an intermediate level or better is invited to join. You use
your own skis and boots; the Park rents helmets. "In two hours, we
have beginners jumping off the 18-meter jump."
The author notes that his group starts off practicing on small snow bumps, before graduating to a 10-meter ramp and ending at an 18-meter ramp.
You will need to be comfortable with downhill skiing (in a straight line at high speeds) in order to have enough momentum for good jumps. If you're starting out on the smaller beginner's ramps, multiple jumps per day shouldn't be too tough on the body (depending on your physical condition).