Currently I am proofreading, so I have to decide which style I am going to follow.
One rule says that you must spell out zero to one hundred and another, zero to ten only.

I prefer to use the numbers, but am grateful for opinions on what works best.

When writing ages, I prefer to write, 17-years old, but the other options are, 17-year-old or seventeen year old?

Thanks.

What tone would you want it to be in?

I mean, in a fiction novel I'd prefer that everything be spelled out, even if it's like... Five hundred twenty-five thousand, six hundred (minutes. How do you measure a year?)

Almost everywhere else, I would prefer the numbers because I wouldn't be there for the style. I'd be reading it for the information, and "numbering it out" actually gets through to me faster than spelling it out. So, I would go with seventeen-year-old.

If it's a fiction novel that takes on a curt and/or businesslike, informative tone then numbering it would be fine.

The key, I think, is just being consistent about it throughout the body of work.

It depends on which of the listed options you're comfortable with. But in my case, I normally use numbers. That only time I hesitate to do so when I'm dealing with 2 consecutive numbers. Say "2 17-year old girls" looks weird. I'd rather spell out the first number or both. English is not really my native language so I'm at a loss at times

Thanks for the feedback. I have decided to go for the zero to ten rule and numbers afterwards. I do use numbers and ages quite frequently as they are markers indicating what era the story is in and opens up the possibility of certain characters knowing others in the same time period.

I also decided to use numbers for money (blackmail amounts) because it felt more relevant and made a point rather than spelling out twenty-five-thousand dollars, doesn't have the same impact as $25k.

I think that when giving an age, or an amount of money; I usually just use the numbers. However when it is double numbers, as in Chan's example above (2 17-yearolds) , then I would also write out the first two instead of using the number 2.   
In this example, the first number (2) represents the amount of people, and I usually write that out, unless it is a larger amount (53 people, for example), and then I would use numbers.
This is kind of a mixture of your examples, and spelling out the 1-10 only, and using numbers after that. I think when it is more than 10, the longer written-out numbers don't look right to me.