By 18 your penis is likely fully developed; however, some guys experience continued natural growth into their twenties. There is no reason you can’t do PE, provided you do it correctly. Many of us here wish we had started at such a young age.
Take the time necessary to read through all of the information provided via the Newbie Routine link at the top of the newbie forum page. You are on your own in the actual practice of PE, so you have the responsibility to educate yourself as thoroughly as possible. Carefully view the demonstration videos to familiarize yourself with stretching and jelqing techniques. Do not fall into the temptation to go straight to advanced PE techniques—you’d most likely hurt yourself. Only add to the suggested beginning newbie routine in small increments, all the time paying close attention to your Physiologic Indicators (PIs—read up on these).
Learn to measure correctly. You can only fairly evaluate your progress by measuring accurately and consistently. Be prepared to practice PE diligently for at least six months or a year to give it a chance to work. No one can tell you how much can expect to gain. Some of us are hard gainers, some easy. Most gains come in a very gradual way. Patience is essential. Practice your measuring several times over the course of a week or so when beginning. If you get the same result, or very close, each time, then you know you are using good measuring technique. Then put the ruler away and forget about it for as long as possible. You probably are best off not measuring much more often than every two or three months. Instead, focus your efforts first on making your PE workouts a regular part of your daily life and on refining your stretching and jelqing techniques. These manual techniques involve a lot of “feel”, something which takes a bit of time to develop.
Something I do to help keep me going with PE is to jot down my workout times in a small, cheap notebook. Each page is dated and filled out with times for warm-up/down, stretches, and jelqs, as well as the number of Kegels I did that day. I also make note of anything else worth considering. I can go back later and see how I added to or changed my routine without having to trust a bad memory. It’s a small daily reward to make an entry in my notebook.
At 18 you probably have privacy concerns, so keeping a workout journal might be tricky. Consider, too, that the newbie routine takes about an hour daily at the maximum recommended workout times, so you’ll need to factor that into your plans.
Be safe. Be sensible. Use lots of common sense.