There is no official upper age limit for an Australian student visa, so your wife being 30 years old is not a barrier. However, to apply, she must meet the academic admission requirements of the chosen course, which usually require having completed prior education a bachelor's degree or equivalent, depending on the course. Since she does not have a bachelor's degree due to failing subjects, she might face difficulties qualifying for degree-level courses but could explore vocational or diploma programs with more flexible academic criteria. She must also fulfill other visa requirements like getting a Confirmation of Enrolment from an Australian institution, passing English proficiency tests, proving financial capacity, and satisfying the Genuine Student requirement. If the academic credential issue persists, she could consider pathways like foundation or diploma courses to then progress to degree programs. Overall, there is a chance for visa grant if she meets these key criteria in another eligible program.