Sounds like you're still using intoxicants though, Mord- just not as much. Why should smoking be any different? Why should the OP have to completely give something up for someone else? Why not compromise?
I don't do anything significantly damaging to my health anymore, our chemical intake is very modest and at parity. Plus I get liver/kidney function tests every 4-5 months.
I stopped abusing my body a long time ago, it's still a matter of debate mind but effectively I have given up a fair amount for her, neither of us indulges in such things purely for the sake of the high, we do it together. I won't get intoxicated in any way while away from her as I just don't see the point.
A compromise wasn't suitable in my case as her habit was not excessive to start, if she bought a pack it'd last a couple of weeks and for that whole time I'd feel wretched. It doesn't sound suitable for the OP either as it clearly bothers his girlfriend to a similar degree. Though it may well be worth reaching a compromise in terms of parity, by having her get over a bad habit at the same time.
Mord- I'm 41 (almost 42), and you've seen my pics.I'd say I'm far from what you've described. There are far more factors involved in the aging process and fitness.
It may well delay things, the fittest person I know has smoked his entire life but he's still starting to get problems at 50.
I may sound über militant but it really cab be just that much of a problem for someone to put up with, it certainly was for me. My girlfriend stopped because as soon as she learnt how it made me feel she wanted to stop. I've only ever seen people quit when they're deep down really wanted to.