Yes he is a US citizen and from a military family.
Is it okay to ask what does marry later means?
No official age limit, but you should wait until your career, desire, emotion and ego have stabilized.
Right now everything is sunny and flowery. Who knows you two will get bored of each other after 5 years of marriage because the desire is strong. Or you two seem to grow apart, more differences arise, your life goals heading to different ways etc.
I guess you have nothing to worry about your current BF, but you still have to beware of the gun culture in US.
Do you have a history of toxic relationship with jealous, violent, cheating men
I just realized that some friends went to my life for example they liked me even though they have a girlfriend or in a relationship. Basically they were mentally cheating to their girlfriends. I unfriended them. 
I guess you will see more of these in foreign lands. Just beware whenever you have to travel alone.