For any bilingual people or people with bilingual spouses, I highly encourage you to raise your children bilingual. When they're still in that language learning phase, not only is it easier to teach them 2 languages, they put up less resistance.
This not consistent with everyone, although if I may recount.
I was raised and grew up in Greece within a bilingual family. From birth when speaking to me, my mother using English(native), my father Greek(native).
By the average age of a child when beginning to speak, I was late. My parents persisted with the English/Greek combination and by about the age of four to five, I was mixing languages when I spoke. Moreover the standard vocabulary levels of a mono lingual child for that age was behind on both languages.
I would make associations with speaking to my mother in English, to my father in Greek, and when my parents switched languages, I became horribly confused.
By elementary school although I had ceased mixing languages, my Greek level was behind that of other natives and children being children, I was teased as a consequence.
It wasn't until my early twenties when I was accepted and graduated from an Australian University, my once thought native level English was certainly not that at all, primarily due to my mother being the only source of English I was subjected to in Greece. Furthermore, English was not generally part of a schools curriculum at that time. (If I recall, French was more widely spoken/popular to speak in Europe about 25-30 years ago)
I had a difficult time understanding and comprehending lectures however, eventually I managed to bring my English up to scratch. (one hopes anyway)
The long and the short of this being;
On paper, it presumably seems natural and logical for a child from multi lingual parentage to eventually have the ability to be native in both, but by no means easy and by no means granted.
Moreover, being a parent within a multi-lingual relationship/marriage etc means that whichever parent is not living in their native country, the role for this parent(s) now becomes parent/school/friends/media and all other sources that shape a natives language that does not derive from merely the communication of a parent(s)
On the contrary, and as from some of the responses here, I have found a large part of children from multi lingual parents are mono lingual, or one of the languages severely lacks.
I personally believe, even though my mother fortunately gave me a great start, I often wonder had I not lived in Australia for a number of years, would my English level be that of what it is currently, as the largest of learning curves was that in fact of variety and not that of single repetition.
To finally tie this in with the OP, variety is something screamed here over and over. Why limit your Japanese studies to only manga, music, movies, video games? etc
I raise the same question when deciding to limit yourself to an age bracket.