Translation:I am eighteen years old.
that's not an exception. it's a actually a rule. try speaking hachisai quickly, it's then easily pronounced as hassai. That's why there is this sign of 'tsu' in the place if you write it out in hiragana.
I typed 19 years old and got it correct with a typo. It ain't correct, it isn't even the same number. I shouldn't have it right.
what if this is the answer to: what is HER age? then juuhassai desu is also correct. desu does NOT mean 'I am'
Depending on the context... But still your answer should be accepted. Because japanese depends very much of the context! '-'
yes, and necessary whenever it would be ambiguous who you are talking about. it's normal to skip the topic if it's clear from context.
'year' and 's' were presented as separate options (with no 'years' available), but 'I am eighteen year s old' was not accepted
There are plenty of comments in this thread that explain this. 八, which is usually pronounced はち (hachi), has an exception. In 八さい, 八 is pronounced はっ (small っ).