What is the most probable cause for weak erections in a 21 year old? the only time in the last year that i, a 21 year old, have had a strong erection

Depends: on multiple factors: stimulation, testosterone levels, stress, tension, anxiety, experience. I assume you are referring to spontaneous erections? Sexual stimulation or masturbation also involve relaxation and enjoyment of stimulation. When in doubt get an examination including blood testing, and thyroid/ blood sugar levels.

Erectile dysfunction: Conditions to consider: diabetes mellitus, HTN, lipid disorders, renal disease, liver disease, vascular disease Endocrine problems such as hypogonadism, hyperprolactinemia, thyroid problems Lifestyle: smoking, alcohol, riding a bike a lot Neruogenic causes Medications.