What Is The Age Demographic Of Internet Users? Preferably In The UK, With A Reference?!

I've used an Internet based questionnaire for my masters dissertation project and could do with explaining why i used the Internet. The reason i did was because i wanted to access a population of 20-40 year olds from across the UK, and it was easier! I figure out of the approx +50% of the population that are Internet users (I've already got that statistic), a fair majority of them would be between 20 and 40 years old!