
I've exported twice to Australia (all other flights have been less than 10 hours, Europe and USA) and the fact the pups were only around 3 months old meant they slept all the way, were not likely to be as distressed about having to dirty their crate as a housetrained older pup might be.
Also as they then had to do a months quarantine they bounced out bold as brass at 20 weeks, being shown within days.
Pups go into various fear stages approaching puberty, where they are much less likely to take new experiences on the chin, than during the optimum socialisation stage, generally agree to be up to about 16 weeks.
I certainly feel that it's not that clear cut and socialisation done at any age with a basic sound inherited temperament will get you a mentally sound dog.
Anyway with the current EU Pet Passport rules pups will not be legally going to, or through EU countries under 16 weeks.
As a breeder I don't think I could face letting a puppy go after about 4 months, which is hard enough, and buyers also want to bond with a pup ASAP..