"I am thirty years old"?
Which is correct?
"Mi havas tridek jarojn"
"Mi estas tridek jara"
There are actually many equally valid ways to convey this:
-Mi havas 30 jarjon. (Ne forgesu la akuzativon!)
-Mi aĝas 30 jarojn. (Ne forgesu la akuzativon!)
-Mi estas 30-jaraĝa. or Mi estas tridek(-)jaraĝa.
-Mi estas 30-jara. or Mi estas tridek(-)jara.
And probably more that I can't think of off the top of my head.
Is the missing dash in 30 jaraĝa intentional? I was just about to comment on the OP that "tridekjara" needs to one word (so does tridekjaraĝa).
All 3 are. The other (arguably most correct) version is 'Mi aĝas tridek jaroj'.
This phrase "How old I am" is intentionally flexible because some languages says "I am" and other languages say "I have". Esperanto is meant to be inclusive for non-native speakers so there is not one way that is correct and another way that is wrong. Just say what you're used to and the meaning will be clear.
While it's true that you can be understood, this doesn't mean there aren't rules. You can see this in the adjective-noun order discussions that crop up here from time to time.
I'm pretty sure that Mi estas tridek jara, while understandable, is also syntactically wrong. In fact I assumed it was a typo and the OP meant jaro, which would be better, but still not great. I'd happily accept Mi estas tridek however :).
I have 30 years is the spanish way to say it (Yo tengo treinta an~os) I am 30 years(old) is the english way to say it They;re both correct