2rights, there is no age discrimination, either positive or negative. You sink or swim on your own merits - that applies to the entire process, from initial testing to final field training. You'll be given exactly the same chance as everyone else.
That said, the training is very full on so favours the speed and flexibility of a younger brain. The generalisation is the older you are the harder it gets, so you at 40 will find it harder than you at 25 (but you still might find it easier than the 25 year old sitting next to you). What matters is it boils down to whether you can meet the standards or not, not your age.
From a purely statistical point of view at 40 you are behind the eight ball.