My kids are 4 and 8 years old.
SINCE birth, my Husband has spoken to them in his native language (he is European). And I speak to them in mine, English.
My Husband is bi-lingual in his language and in English.
I am not bi-lingual. I only speak English.
My kids are totally bi-lingual in speaking and hearing and for my eldest, in writing.
They are competent on both languages and it does NOT impair them in any way
In fact, being bi-lingual, to me, helps them to acquire other skills, more readily. And it has been proven, that being multilingual builds more synapses in the brain... as a child develops.
My kids for example, really pick up on other languages very easily, and the accent for it.
My eldest child, also knows Mandarin, and Hawaiian and Japanese. Which is taught at her school.
We never applied any formality to it or rules etc.
We just talk to them naturally, in our own languages.
That is how kids learn.
Kids are sponges and they learn.
It is NOT IN ANY WAY, detrimental.
Some countries, are just naturally multi-lingual, and it is never a bad thing.
It is best, if a child learns it when young and naturally. Per their parents. In the home. In daily life. Just ordinary daily life.
That is how they learn.
As a person gets older, it gets harder to learn another language.
As I said, my Husband and I, since our kids were born, the very first day they were born... we just spoke to them in both languages.
They fully know... and understand it. And, they have a greater appreciation & openness for other cultures, Because of their bi-lingual exposure.
*As a side note: my son was delayed in Speech. It was really no big deal. Many boys are. Our Speech Therapist said, it is in NO way, due to his bi-lingual languages in the home. If anything, she said it is a plus.
My son, although he used to be speech-delayed... was very articulate.
Speech-delay did not affect that.
My son meanwhile, is now the MOST talkative one in our family.
AND his vocabulary & understanding of words/ideas, per his age... is much more ahead than other kids his own age.
It is NOT "confusing" for a child to grow up in a bilingual or multilingual home. That, is a fallacy. Not true. I don't see why, people think this is a bad thing.
It is... most positive. To me.
In many places, people grow up Multilingual. And it is, advantageous.
By the way, I let my 8 year old Daughter, read your post, to see what she thought. She, being bilingual and grew up that way.
She thought, it was silly to be afraid of teaching a child to speak in more than 1 language. She said "that is the way you are born... and how your parents are and speak... it is not a bad thing.." She said it doesn't matter if you speak 2 or more languages at home.. your kid just learns.
She was surprised... to see that people even worry about it... teaching a kid more than 1 language. Because to her, it is commonplace and "no big deal."
My kids, have NEVER been 'confused' about their languages. At all. They are fully competent and much more articulate, verbally and conceptually, than other kids their own age.
To answer your Question:
1) my kids, have been spoken to in both languages since birth. They know no different. But are proud, of what they know and speak.
2) My Daughter, as most girls are quicker in speaking, started talking a bit before 1 years old. My son, at about 19 months old.
3) No. We always used both languages in our home. No biggie. My kids speak English well. And the other language too.
4) No, learning 2 or more languages at one time, is NOT a 'detriment."
5) To me, a child should learn both languages, at the same time. Meaning, there is no reason to stagger it. My kids are fluent in both.
The only concern I have is this: IF a parent speaks to their child in one language, but in parts of the same sentence uses the other language too... THEN your child will learn the WRONG way to speak. Because the sentence/grammar structure will be wrong and it will then be a 'pidgin' version of the language. Not the correct way.
all the best,
Susan