I'd say Finnair, though at least get to LT Silver with QF at some point. Finnair's program just changed, so I can't imagine them moving the goalposts anytime soon.. but the difference between QF and AY lifetime is that AY lifetime Plat is not too bad after reaching Gold.. whereas QF lifetime Plat is ridiculously far after Gold and is not worth the effort, even at your age. You have a better chance of making LT Plat with AY at some point, even if you are satisfied with LT Gold.