You should say I am X years old, however, that doesn’t mean you’re old at all :)

Thank you Gray.I prefer to use I am...young than old. :)

Lisa's example is good. People usually use "X years young" when they are making a sort of joke about how "old" they are. A person could also say "I'm 30 years young," but just as in Lisa's example, this is a way to jokingly make the age "30" sound "young" (which shows that the speaker probably doesn't actually think 30 is a very young age).

Thank you.

Not really, but it is used in one context. Older people might say "I am 70 years young" as a way of stating their age while emphasizing that they feel younger.