For a successful marriage you’ll need maturity,confidence,decision making,communication and compromise skills. Each person is unique and I can’t say that someone young 18 yrs old is any more ready than someone 23 yrs old.
But the younger you are the more emotional, financial and family support you’ll need aside from a supportive, mature,happy,stable husband.
So I would say don’t date at 18 yrs old (could still be in 12th grade though finish up high school if necessary)
I tell girls to read Frum dating books to prepare yourself, go to seminary-you’ll have Chinuch,Marriage classes. It’s never to early to go to shuirim and you could listen on Torahanytime.com
Basically 18 yrs old isn’t too early as long as you’re well aware of what marriage entails.