I’m 18 as well. I was told for many years by people who either used to have a stammer or knew someone with a stammer that “oh don’t worry about it it’ll go away before you reach 18 or so” and here I am about to turn 19 and there’s no signs of it even getting any better. I think once you reach an age where a stammer starts to become a real problem (job interviews, making new friends etc.), speech therapy can probably go a long way. Obviously it’s entirely up to you and your situation is different to mine, but I reckon the soon speech therapy is started the better.