There's no such thing as a perfect time, but I believe from my own experiences that the sooner, the better is true. I kept thinking that when I got older and had more money and less issues with bullies that it would be easier. Well what I found out is that there are always bullies, no matter how old you get. Also the more money I have, the more financial responsibility I have, and therefore the less money I have available for a transition.
The longer you go before transitioning, the longer the list of people you need to come out to gets too. It won't be just your family and current friends. It will also include new friends, coworkers, bosses. Let's not forget all the bills you'll wrack up. For instance medical offices, rent/mortgage, utility companies, insurance, credit cards, car payments, driver's license, discount cards for stores, banks, your company. That's a lot of people to need to come out to, not to mention having to contact your school after you graduate to get them to update your name so you can prove you have a degree. It would have been so much easier in my opinion, if I had done it in college so I started life in the real world having already transitioned. At 18, you also haven't hit the full effects of T yet, so you still have a chance to beat some of it out.
You also start to form opinions about how the news will be received. My opinions were all wrong. Everybody in my family that I thought would be accepting, walked out of my life the day I told them. The few I thought would have a problem with it, are still in my life. I honestly didn't know how my friends would take the news and every single one of them not only still talks to me, but they still go out into public with me too. But I spent far too many years dwelling on my incorrect expectations. All that could have been cleared up decades ago.