Hi! use native korean numbers for age, not sino-korean. So, use 열네 on this one. But, make sure that you use the counter word for age after it (as in, 열네 살) because of the way some numbers change depending on whether or not they are immediately followed by a counter.

It depends on what word you use for "years (of age)." As the answer above noted, with '살' you use the native Korean numbers. If you use '세' (Chinese 歲), then use the Sino-Korean numbers. So 열네 살 but 십사 세.

열네 if you want use 살 for 'age'. i think the word 살 is more common