For example, if I had been creating Blarth, a conlang, and I had a question--could I ask that question here, or is it limited to conlangs that have been around for a while?
From the Tour:
Constructed Languages Stack Exchange is a question and answer site for designers and users of artificial or invented languages
So, yes, questions about problems you are experiencing creating your own conlang are definitely on topic.
However, if you want a good answer you'll have to provide enough context so that people unfamiliar with your language (just about everyone!) will be able to answer from the information you've provided.