Difference between 几岁了 and 多大了 I googled a bit on it and found different explanations. Some says it just depends on the region and dialect. But my text book explains that as a rule of thumb 几岁了 should be used for kids before 10 years old, and for other cases 多大了 is Ok. Could you please provide some more explanation on it?
Your textbook is right. But usage might vary. 几 generally means few, the convention that its maximum is 10, that's why you can not ask someone looking older than 10 你几岁？ 

Of course you could ask a little boy or girl 你多大？, that's not wrong, but do not ask an adult 你几岁/您几岁？, this wouldn't be appropriate. 

The only context when 几岁 could apply to adults is 50几岁 (fifty something of age) and, of course, 30几岁,40几岁 and so on. Here 几 only refers to the 'something', the unknown few years someone is older than 50, etc.