HELP!

So for the past few months, my boyfriend of 2 years has been talking about getting engaged. Because he went into the military recently and he’ll be leaving the country soon, it’s been getting REALLY serious. I’m 20 years old, a senior at my university, and am confident enough to take this on after I graduate. BUT my parents have other views. They think I am too young. Even though I see their point of view in wanting to protect me, I really don’t want to spend 4 more years having a long distance military relationship. How do I respectfully tell them that I’m ready to make this decision?