The most recent set of figures from the DVSA (2008 in a major study) is that the AVERAGE is 47 hours of professional tuition plus 20 hours of private practice. Whilst there are many people who do it quicker than that, that figure is the average so there wil be people who take a lot more!
Being older does affect the learning period so being young would suggest that you will learn to drive quicker. However, slightly older learners, motorcyclists and cyclsts can have a better understanding of the dangers and interacting with other road users so they make up time where young drivers might take longer.
Furthermore the roads are busier now than they were then and the standard of driving is generally deteriorating (simple observation) so the average may have now increased.
When I start with a brand new pupil I always point out the average and the likely cost so that they have a good understanding of what might be required. Most of my pupils pass after around 35 hours but not all as it is entirely down to the inidividual. It also depends whether someone is learning to drive or learning to pass the test. Whilst the latter is usually the goal, there are some instructors out there who try to take you beyond a minimum standard so that not only are you good enough to pass your test but you can also handle almost every situation that you are likely to encounter. This may seem an unwarranted cost but I, and some of my professional colleagues, believe that driving safely should be the goal and not just passing the test. If you do the former, you will naturally do the latter. Unfortunately sometimes the test does not do the job properly!
If you are brilliant enough to pass with just 3 lessons then great but that is the exception rather than the rule.
p.s. Not doing your theory early enough can also string out the lessons unnecessarily.