I think the key word in this entire argument is...potential. Superman, at 18 years old, you still have the potential to gain more size naturally. I'm assuming you are just like me...the classic hardgainer. I have a very fast metabolism. It is extemely hard for me to gain muscle, and very easy to lose it.
When I was 18, I weighed about 170 lbs. I'm 6' tall. I didn't start using steroids until about 3 years ago. I'm 34 now. When I started using them I weighed around 235 lbs off-cycle. I'm not saying that you should wait until you're in your 30s to start using. My point is, that I am glad that I waited some time before using them, because I gained about 65 more lbs naturally first.
Take the pro bodybuilders for example. If they were to stop using steroids, they would not shrink down to 200lbs, they would still be very big. That's because they all waited until they reached their natural size limit. If you start using steroids now at this young of an age, you are highly likely to end up very disappointed. Steroids have their own limits too. You will do your first cycle, then most likely end up right back at the same weight you were when you started.
Anyone can gain weight, no matter how fast their metabolism is. Like Grizzly and others have said, if you are not gaining weight, you need to make some adjustments in either your eating, training, rest or all 3. You also need to be sure you are trying as best you can not to do anything that will put you in a state of catabolism.
Another thing is, you said that you gained about 30 lbs in a relatively short period of time back when you started training. Well, that happens with all hardgainers. It is very typical to gain 20 or 30 lbs very quickly in the first year, then suddenly finding that gaining just 5 lbs afterwards seems like an impossibility. That is normal. This is when it's time start making the adjustments in the factors I mentioned above.
Gaining weight for the hard gainer takes a tremendous amount of dedication, hard work, determination, SANITY and more than anything else...knowledge.
Wait for at least a few more years. You have a long way to go. I hope this helps bro. I was where you are at now. I know just how hard it is.
KB