I'm generally very against anti-libertarian laws, but I think what sways me with this one is the clear fact that so many older smokers wish they had never started and counsel younger people never to start.
I think this is a good way of introducing the law. Like it or not, there are loads of people who are already seriously addicted, and you simply can't withdraw their 'grandad rights' from them now, without a whole load of negative effects, especially considering their addiction began completely legally.
Yes, younger people may indeed get older people to buy fags for them; but the idea of this law is that it's for the long-term. If it's brought in, the number of people who can legally smoke (or indeed buy them on behalf of younger ones) will gradually grow smaller and smaller, until eventually there will be nobody alive who can buy them.
True, banning things doesn't completely eradicate them, but it avoids sending mixed messages from a government saying "You really, absolutely must not smoke... but it's totally legal if you do". It does add another step for interested people to have to overcome, and a lot will just not bother, if it's not out there openly available in shops.