I think researchers agree that the older you get, the harder it gets. So that there are age effects.
So starting early is good, in principle, but only if there is enough language input, a rich language environment and various preconditions are fulfilled.
But there is no clear age coming out of the research, after which it is no longer possible anymore to learn a language. At least not a second language.
I think it might be different for learning a first language. Then there are several indications for a critical period.
But for learning a second language, for example as is the case when offering sign language to your child, I would say as early as possible and as much as possible.
So this is just my opinion: I think that if your child doesn’t pick up enough or not much of the first language...
then it doesn’t matter how much more you are going to talk to them in that language. Some things will always be left out or will not be picked up. That’s what it seems like to me.
So it’s a good idea to offer a language to which they do have access. It can't hurt, let's put it like that.
And if you choose only to offer spoken Dutch? Yes, that will work out fine for one child, because he or she follows everything.
But for another child, this may of course not be the case. And that is very unfortunate, because they might have problems...
they have to go to school, if their language proficiency isn’t up to scratch.
So I think if you have the option of offering a language to which they could have full access...
I would do that. When we look at bilingualism with two spoken languages:
it is not the case that when I offer more input in one language, the proficiency in the other language will get worse.
It’s like I just said: when you look at the amount of language input, it’s the more the better, but only to a certain extent.
And what we see in the research is that it doesn’t matter if you hear the language 70-80-90-100% of the time.
You can ask yourself what monolinguals do with all that language, but that’s a good thing. And it has to be like this. Otherwise we wouldn’t have any bilingual children.
So based on that reasoning, I would think that you do not necessarily have to spend all your “language time” on Dutch.
But yes, it’s of course more difficult if you do not know exactly how much input your child is getting.