For the most part none of this information will actually be directly shared with your readers, but the very knowing of it will make your character seem much more solid when you write about their actions and dialogue.
Personally, I like to push myself into going into as much detail as possible by breaking it down into the following sections:
- 20 - 30
- 30 - 40
- 40 -50
- 50 - 60
- Etc - up the age that they are, obviously.
You may feel this is overkill, but creating characters that feel like real people is one of the hardest parts of writing a novel, and when you take the time to get to know your characters in this much depth, they begin to take on a life of their own.
For example, when thinking about the childhood of your main character, you note down that they once fell into a bush of stinging nettles, and now they hate walking through the woods. The stinging nettle story may never come up, but if your character is forced to walk through a forest, they might react nervously. On the other hand, another main character, who spent their childhood climbing trees and being active, relishes the walk through the woods.
Of course, you could have created this conflict and contrast from thin air - i.e. just decided with your godlike powers that one of them likes the woods and the other doesn't - but knowing why just makes it feel more solid, and that will show in subtle ways in the prose.
Furthermore, as you're forced to think about each stage, you'll get a better grip of their career trajectory (or whatever) and you'll get to know more about the peripheral people and influences in their lives, such as their first boss, the guy that sits next to them in the office, their first crush on that Russian. It forces us to justify the elements of their lives - such as why they live in a two bedroom apartment. Does that actually fit with their parentage and current earnings?
Another advantage of this is that you'll find incidents occur to you that really will end up informing their behaviour throughout your book, and some of them may even make it as scenes - but be careful of including a scene from their childhood just because you've become emotionally attached to how awesome is.
Even worse, don't even think about giving a low down of the history of the character in chapter one.
Knowing your character's history is your business, just like knowing your own. Elements of it will naturally come up in conversation with people at appropriate moments, but you wouldn't meet someone and immediately tell them your life story. Keep it subtle and your characters will come across as real people with full lives independent of this particular story that they're currently taking part in.