The Guardian’s Data Blog has taken a look at the Drug Misuse Declared data and makes the observation:
The most common age for first taking either powder cocaine, ecstasy or cannabis was between 16 and 18 years old. The most common age for cannabis was 16 years old whilst for powder cocaine and ecstasy it was 18 years old. The report also found a wide variation for age of first drug use from as young as seven up to 57 years old.
The raw data for this is available here. One of the things you notice if you dig into it is that there is quite a difference between when older people who have used drugs remember first using and what those under 35 said. It is also worth noting that the youngest age group – 16 to 19 year olds – will inevitably have started earlier.