Does maintaining a healthy lifestyle prevent erectile dysfunction and improve sexual performance in an 18 years old male ?

Here are some ...: Yes, healthy lifestyle with no overindulgence & obsession will definitely help improve & maintain the erectile function as long as possible along the life, but there is still a limit for everything. For example, a 18-yr man may run into ED if he wants to have masturbation or sex 3-5 times in 1-2 days, and I have seen such men before. So, regardless of age, try not doing it > once every 1-2 days...

Yes, but generally: this is in the context of a much older male. The risks for ED are those of any other circulatory condition: smoking, blood pressure, blood pressure medication, cholesterol, diabetes. But by far the biggest risk (8 fold increase) is lack of 200 calories of exercise per day. Hope this is helpful. Never too early to start healthy habits.