The Chlamydia Screening Programme have a target of screening 25% of 16-24 year olds. This is a HUGE challenge and they'll do whatever they can to try and reach that target...a mail out is an easy way to reach high numbers of young people.
It is estimated that at least 1/3 of young people are having sex before they are 16 years old, however many more than that may be sexually active in the sense that they are having sexual contact. Chlamydia can be transmitted through any form of unprotected sexual contact (this does not have to involve penetration) so a lot of people who think they haven't been at risk, may have been. When you count your sexual partners, you should include anyone you've had sexual contact with - you don't have to have had "full" (penetrative) sex for it to count!
A lot of young people want to take a test but are too embarrassed (there are many other barriers too!) so postal kits are an ideal way of reaching those young people. The Chlamydia Screening Programme should probably think a bit harder about the potential consequences of doing this. Having said that, they are likely to reach out to many young people who couldn't bring themselves to go and get tested elsewhere (for whatever reason - whether cultural, religious or whatever).
Chlamydia is sooooo easy to treat if you detect it early on (a simple course of antibiotics) but can have severe complications if it goes undetected for a long time. If you get the opportunity get tested, all you have to do is pee in a pot and you get peace of mind!
Asim18: maybe you could just explain to your mum that you didn't request the pack and that they've been sent to all young people in your age range? I know it's probably not that simple. Good luck though.