Its called premature ejaculation,very common in young adults and normal. Mainly its due to overstimulation mentally or overexcited .It is not related to size of penis.You need to calm down and engage in foreplay and understanding partner more rather then just rush into penetration.Repeat this and over period of time body will get used.Also you can try stop and withdraw movements.just before you know you are going to cum stop doing the activity.Even while masturbating stop doing the movements before you cum then relax and start again slowly. This way time taken to ejaculate is longer and lasts longer in few patients. If you are very anxious person and nervous this can happen so mentally you need to calm down.