Learn Romanian: Bărbatul și băiatul sunt aici.

In Romanian, the word for 'the' (the definite article) attaches to the end of the noun! Since bărbat (man) and băiat (boy) are masculine nouns, we add the suffix -ul to make bărbatul (the man) and băiatul (the boy).

I learned that sunt means 'I am'. Why is it used here for 'the man and the boy'?

The verb a fi (to be) has a special quirk: the form sunt is used for both 'I am' and 'they are'! Here, the man and the boy make a 'they' plural subject, so sunt translates to 'are'.

If we didn't mention the man and the boy, how would we say 'They are here'?

You could simply say Sunt aici. Because Romanian usually drops subject pronouns, you leave out the word ei (they). The verb sunt alone is enough to show you mean 'they are' when the context is clear.

Does the word aici (here) ever change since we are talking about more than one person?

No, aici is an adverb, which means it never changes form. It stays exactly the same whether you are talking about one person being here or a hundred people.