Can an 18 year old active male become a vegetarian? Is that healthy?

Vegetarian. Sure. As long as it is done in a balanced, thoughtful way. Just remember that if you cut out meats that you need to still get certain nutrients that are mostly found in animal products (although easily found in non animal products, just takes research): Omega-3-fatty acids, Vitamin B12, Calcium, Vitamin D, and iron. Follow the link