Juliet was 16 like Samantha and her story is a classic that's survived for hundreds of years as the pinnacle of romance. No one claims Juliet was too young to fall in love.
That being said, I agree that Samantha took a huge emotional and physical risk getting involved with Delaney. He is a troubled character, a product of hook up-culture -- arrogant and flip, gorgeous and brash -- nothing like prince charming. He and Samantha crash into each other, sparking a chemical reaction that allows them to become something new, leaving behind all the crap young women experience in our society, including slut shaming, sexual harassment, gender roles, and homophobia. The openness and lack of traditional boundaries of Samantha's parents make it possible for her to navigate the strong connection she feels with Delaney.