Many people seem to think that this is a joke questions, but it is a real issue for people looking for a sex filled career. Requirements are different for men and women. Women are in great demand for a veriety of looks and beyond being over 18, they need to really enjoy sex and be comfortable with their own bodies.
For men, you need to be comfortable with your niche acts, be long lasting, be able to start and stop (that's a hard one) and be able to control your spout. Gay friendly talent is in high demand. Being a male po*n talent is considerably more competitive, challanging and difficult than for a women. If you can't get it up one day, you will very likely find yourself out of work...perhaps forever. After all is the fluffer needs to come by every few minutes, the rest of the crew will be waiting around costing the producer money.
There is also the possibility that you won't find the situation or partner attractive and not be able to perform. This is also the case for women, but isn't as noticable.
Lots of people seem to think that you need to be attactive to be in po*n. That's not really the case. There are a lot of fetish niches that need to be filled (fat, flat, glasses, big/small, etc.).
It be a star is a little different. Stars fit a couple of models. For women, its about being beautiful, blond and bountiful. For men is about the monster cock (that's where Ron Jeremy comes in). Having a toned chest and a 6 pack, won't get you there as a male po*n actor. You'll need something more as it is rare for men to become true stars.
po*n today isn't really about star culture like it once was. I think this question was asked by a women so I'll leave you with this advice. If you are going to do it, plan to get everything you want by age 25, and do what ever it takes to make it happen. Think of it as "freedom 25". But remember the bar is going up and there are more sexually liberated young women coming up behind you.