How many of them really take drugs?

It's easier for young people to come into contact with drugs nowadays than it was when their parents were young.

But it doesn't mean they're all at it. Most young people are sensible. Many experiment with drugs and then stop. Some experiment, continue taking them through adolescence, and then stop. And some become regular drug users.

6 out of 10 young people do experiment with drugs, usually cannabis. And they're starting younger:
1 in 12 of 12-year olds have tried drugs at least once, as have 1 in 3 of 14-year olds and 2 in 5 of 16-year olds.

Smoking and alcohol use are widespread among young people, starting at around age 13-14 on average.

One serious problem is the large numbers of young people involved in binge drinking. And being middle class, white and/or female is no protection. Women are outstripping men in the amounts of alcohol drunk in some areas.
One issue is the relationship to cultural norms. Alcohol is acceptable in law, and ecstasy isn’t.
In the 1900's it was socially acceptable to use opiates in patent medicines for both children and adults.
Many notable figures are thought to have been addicted to preparations such as laudanum.

A growing number of children are brought up in households and communities where drugs and alcohol are openly used.
For more detailed information and statistics on drug taking amongst young people,
see How Many of them Really Take Drugs? The detail