Yes, and then you'd have all the complaints about somebody's lovely old granny being cruelly forced to give up her lifelong pleasure in her last few remaining years.
It also occurs to me that, if a huge amount of people were legally able to buy a substance for many years that they then became thoroughly addicted to - especially those who began before the health warnings on packets and the ban on advertising - and which they were suddenly legally prevented from buying anymore, there could be an absolutely colossal legal class action against the government in the making.
If smoking IS to be banned - and this seems to be the will of the government and most adults (including a great many current smokers) - then what other practical, fair way is there of doing it, other than to allow existing adult smokers to retain their right but not to extend that in perpetuity to future adults?
It's completely normal for things to change as new generations are born and old ones die. Some changes may be widely seen as good for younger people (e.g. no longer having to do National Service) whilst others maybe not so much (e.g. having to pay for university tuition).
I suppose there will come a time, once 100% automated driverless cars are becoming the standard, when older people with driving licences will be allowed to keep them but then no future ones will be issued to younger people reaching 17.