I am 18 years old and i still have early preteen breast development?

Probably normal: You may just be a late bloomer. A lot depends on what age you started having periods. If you have pubic hair, acne, and nipples which appear normal, your hormones are working. If you are too thin, that will affect breast size too. Half of all women have breasts which are smaller than "average". Don't let your breasts define how you feel about yourself. If still in doubt, see your physician.

Needs clarification: It is not clear if you barely started a breast development or its development halted at same stage for a long period. In either case, you may need thorough evaluation especially if you have not developed subsequent pubertal events, e.g., pubic hair, growth spurt, and menstruation.