There is no requirement of age from Uni, however, from Visa perspective you're in the higher risk range. as you're in 40s, unless your personal statement regard your future design can give the visa officer a good reason to believe that you're a geniue student to Australia. and you're not going to stay in Australia after your graduation.
there won't be any offical documents for this info, it's only the understanding of their policy or common scense. they won't put it into writing by not issue you a visa just because of you're 40, it's discrimiation by age. they only can not issue people visa if they think it's high possibility this person will abuse this visa and is not geniue student visa holder. you better to counsult a qualifed consultant to design the best solution / plan for your migration / study to avoid wasting time / money.