Hi, 'm Charlene 21years of age and my menstruation is irregular ever since, I am now more than half a year delayed. I would like to know the possible cause of this. Thanks in advance!

Ever since when? More than 6 months without menstruating at all?
Possible causes:
Hopefully just this one: some women need other menstruating women around to stimulate their own.
OTHERS
Poor nutrition, especially lacking iron.
Extreme athletics
Unusually slow sex hormone development, (called immaturity of the hypothalamic–pituitary–ovarian axis). Puberty onset can vary a lot, and it can be irregular at first. Age 21 is late though.
Low thyroid hormone production.
Medical conditions around the uterus, such as PCOS. Generally you would have pain. One sign might be a fair amount of hair growing above your lip or around your face.

If you can have a doctor give you blood tests and an ultrasound, that would cover some of those. If you are trying to get pregnant, try to be patient, or see a fertility doctor. That can get expensive.