So, i've noticed that my first bicuspid is loose, but read online it's not supposed to be like that. is it normal if it falls at the age of 15-18?

Entirely possible that what you have is an over-retained lower second deciduous (3 year) molar, a baby tooth. It may or may not have an underlying second premolar. Please see your General Dentist or a specialist Orthodontist for evaluation and space management.

What you've read is correct.. at 18 years old you should have all of your secondary teeth,and they should not be loose. Why not have your local dentist take a look and set your mind at ease.