New to the forum eh? Many times folks don't bother to read carefully but just plunge in with a reply, whether that's what you wanted or not. On the other hand you could have stated up front what you knew the answer to be instead of letting people guess whether you are a newbie or not. Just my 2 cents. ;)
FWIW many times the trailer tires will deteriorate with age-related sidewall cracks long before the thread wears out. Rule of thumb is to replace automatically by 6 or 7 years from manufacture, or cracks appear, or tires wear out, whichever occurs first.
Yours could be due to any number of reasons but since it is a single big crack (to me it looks more like a stab wound but could be pinch damage since there are smaller cracks) it is likely not age-related.