I think you are very wise to wait, and I agree with telling people you are waiting until he comes back from the army. Should be good enough to keep their mouths shut.
But wow, where I live the mentality is quite the opposite! If someone asks an 18 year old when they're getting married, they'll get a funny look and be considered crazy. My cousin is 19 and extremely mature and she was lucky enough to meet the love of her life last year (whose also 19). They are both in college, although my cousin studies half the year and then works the other half for the government (a simple clerk job) to have money for school and what not. They are getting married this summer and many of her friends are against it (she's lost many friendships due to this), but they are both ready for it in every way. My sisters are also not too happy about it, but it's because they don't know her as well as I do since I have spent much more time with her (we are the same age, but my sisters are 10-11 years older than us).
Anywho... they're probably all asking because they are in the mood for a nice celebration and some good food and dancing.
Well, good Lord..If they're looking for a good party, we could always have a bonfire and cook bbq on the grill! ;)