I guess it's 18 for girls and 21 for boys by constitution. But your question is not so clear.. what you want to know as an average prevaling age of bride and groom in nepal or  age by constitution.

Anyways prevaling average perfect age for a guy is 28 and for a gurl is 25, however that are many factors that affect this age. Like, your education, job stability, family acceptance and also the place you stay and your and your parents perception about what is the perfect age for marriage.

Dont know if this helps you or not and don't even know what exactly you are trying to know.. anyways if you are planning to get married then good luck !