Imagine turning 18 (or whatever the smoking age is in the UK) and starting to smoke during the year this rule takes effect. Then, every year from that point forward, you'd have to wait for your birthday to start smoking again.
Most people who "start smoking" don't just pick up a pack and BOOM addicted. It's a psychological conditioning over many months or even years that leads to full on addiction. It starts with a drag or two off a friends cig, then it's a pack purchased for weekend partying, then it's "I'll have a cig with my coffee", then it's "I smoke half a pack a day". A public awareness campaign coupled with the "one more year" approach to smoking laws would basically eliminate all new young smokers. Vaping on the other hand is actively being pushed onto kids. Kids don't want actual tobacco anymore.
Thanks, I'm well aware of these issues. My point is that they should just prohibit what they think should be prohibited, and that they should do so for everyone, not just those that sit below an arbitrary age threshold. The sole point of this proposal is to pass laws selectively so that they don't affect age groups with whom they would be unpopular, and I don't think this is how laws should be passed.
If a rule were implemented it would most likely be "no one born before 2005 can purchase cigarettes.
Then 18 year olds born on January 1 or December 31st of 2005 can purchase cigarettes for the rest of their lives but if you were 17 when the rule was implemented you're banned for life.