如同starskipper所说，这句话是美国俚语。
当我们说一件事如果“总是”（always）那样，说明不但现在（are），而且将来（will）也会一样。

要看语境（上下文）

grammar
The "will" here makes the sentence future tense. "Boys are boys" would also be a valid English sentence. The difference is that the "are" makes that sentence present tense. As far as the grammar is concerned they are both perfectly valid; it just depends which tense you're talking about.

meaning
"Boys will be boys" is a well-known English idiom, which happens to be phrased that way and posed in the future tense. The meaning of the idiom is that you should expect boys to misbehave, so don't be surprised when they do. (you should expect (future) boys to misbehave, so don't be surprised when (in the future) they do.)

I'm just guessing here, but I suppose the reason it is phrased in the future tense is that it implies that it will always be the case, lending weight to the original point that there is nothing that can be done about it. Boys will (always) be boys.