pension tax relief changes at 40 years old

I will be turning 40 later in the year. I have been contributing to my work DC plan the maximum 20%.

At 40 this will change to 25%, is this relief prorated for the year or does this apply to the whole of 2018?

It's the whole of the year.

Life begins...

The maximum amount of pension contributions in respect of which an individual may claim tax relief may not exceed the relevant age-related percentage of the individual’s earnings in any year of assessment.

So, if you turn 40 at any point during 2018 you can claim tax relief on pension contributions of up to 25% of your net relevant earnings (essentially your gross income from employment or self-employment, subject to a ceiling of €115k).

Bear in mind that you can make relievable pension contributions relating to 2018 at any time up to 31 October 2019.