It is counted from the 1st of January following your 25th birthday. i.e. your 26th year (as stated above by Lob Rockster).
Those born towards the end of the year lose out.....
Unfortunately there's no way of them calculating premiums in direct relation to people's birthdays. Imagine doing this for 6 million people?!
They had to come up with some ruling, so this is it.
The worst is for those turning 19 where your premiums go from about 70.- per month to 170.- ! (or more depending on where you live)
Look on the bright side, at least your car insurance will go down