GDU, there's no age limit and you need Year 12 or a pilots licence. Check out the Airservices webite for specific details.
There is no age discrimination (positive or negative) so you have to sink or swim on your own merits and the learning is fast paced and long. There is limited scope to cater for those who are a bit slow on the uptake - something that comes to all of us as we age. At 45 you are mentally slower and less flexible than you at 25.
That said, your performance is measured against a standard so you either meet it or not. You at 45 might well be more capable than the next applicant at 25, so your age itself is not a show stopper. It just makes it harder.