Difference between 几岁了 and 多大了
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.