Not all retirees. Some even use proper English grammar!
It will be hotter and more humid in Cairns.
Suggest you do some research on both to see which suits you interests better. IMO, completely different experiences, although Port Douglas could be said to resemble Noosa Heads in its laid back, upmarket casual aspect. And both have beaches. Cairns does not.
Surely if you’re coming to study, the college or university matters more? Have you researched where you can get a place for a course that you want?
What kind of visas do you have?
If you want a backpacker scene, then if looking at the Sunshine Coast (of which Noosa Heads is part) then consider Mooloolaba. Sunshine Coast offers surf beaches, lovely hinterland, access to Fraser Island.
Up to you whether you want surfing or snorkeling.
If you are planning to work ( and have the appropriate Working Holiday Visas), it makes sense to go where there is a shortage of your particular skill sets. Cairns has a large transitory backpacker & gap year population.