Asking someone's age
There are several ways to ask someone's age and they vary from neutral to very polite. "他几岁" implies that the answer is within 1-9 range, since 几 should not be used for numbers greater than 9. 

As for other ways to ask the age question (all of them apply to adults):


你多大? - neutral
您多大年纪? - polite, usually used for eldery people
您贵庚？ - very polite and very formal