Can a girl become a great professional ballet dancer starting at the age of 18?

nothing is impossible but you'd be starting at an age where there's already professional ballet dancers.

Yes, but they are recommended to be a certain weight and height. But with enough practice , deterimination, and perserverance, you can do anything you set your heart, mind, and prayers to. 

In spite of what others have said, the real truth is you are too old. Don’t feel too badly as most who start young also don’t reach the professional ranks. There are so few jobs and many gifted and well trained dancers ready to fill those jobs. The main problem is you have aged out of the full time, world class professional level training, that is required for a career as a dancer, I don’t see how you can do this no matter how gifted you are. It takes ten years to “make a leg” in dance which gets you to the age that many professional dancers start to retire. Which is why you have aged out of type of training. That being said, no where is it written that you have to get paid to enjoy dancing. Recreational dance is for anyone of any age, shape and abilities. Find an adult beginner dance class and get started.,

It's possible but unlikely starting that late.

It's definitely within the realm of possibility, but it would require a massive commitment in terms of time training and of mental effort. The rule of thumb we use where I teach is 7 years of serious training before we recommend starting to audition, but that number goes down with talent and it goes down much more if the student has other dance training or training in other physical disciplines like martial arts. It's more common for men to advance quickly, but I feel like has more to do with pointe work than anything else. Pointe will be the biggest hurdle for any young student pursuing ballet as a profession.

It’s also faster for men because there’s so much less competition. Smaller non prestigious companies literally advertise that they will take on men who only have athletic or theatric experience with no dancing.

So to answer your question Yes!! I just started ballet and I’m in my early 20s. My instructor told me not to feel bad about being so old because she taught another girl who started as an adult and went professional. My instructor also started as an adult and now teaches several classes. She told me that if I put enough passion into it, I could go professional, but it’s a hefty commitment. It takes a lot of time and discipline. It's never too late. Go for it!!

Simply put - no. 

If by "starting" you mean 18 is when she first starts learning ballet, then no.