New to Mandarin and need help with a translation

[Serpenta]

I've just started studying mandarin with Rosetta stone. Right now I'm currently working on unit 2. There are two phrases which see to mean the exact same thing to me and I was wondering if i could get some help distinguishing them.

Nǐ jǐ suì?

and

nǐ duō dà?

They both seem to ask age.

Thank you for your time

[fanglu]

The both ask the age of the listener and are most appropriate for asking a child. There is no substantive difference.

[rueyshyan]

nǐ jǐ suì which literally means "You how many years old"

nǐ duō dà which literally mean "You how big"

[li3wei1]

I was taught that the first one is for children, and the second is for adults. I don't think it's a horrible mistake to make, though.

[tooironic]

几岁 is used for asking the age of someone you expect to be under 10; 多大 for people you think are over 10. This is because Chinese has two words for "how many" - 几 and 多少. The former is used for numbers under 10, the latter for numbers over 10.

[andysun731]

你几岁 and 你多大了 can be both used to ask someone under 10, like what fanglu said, no substantive difference.

你几岁 How many years old are you

你多大了 How old are you

[skylee]

Also consider 您貴庚? nin2 gui4 geng1 (for older people or to show respect).

[Lu]

In Taiwan, 你幾嵗 can also be used for asking the age of adults. (Not greatly relevant for the OP, but if you encounter this, you now know not to be a little offended, as I was.)

[Serpenta]

Ah thank you for the responses. I appreciate it very much.