Macpac's baby carriers are made with the same superb care and attention to detail as their Rucksacks. The results are robust baby carriers that are comfortable both for those carrying them and those sitting in them, and that will surely become family heirlooms of the future.
Backpack type baby carriers are not suitable for children under six months of age as they have trouble supporting their necks for long periods prior to this age. They are never suitable for high mountain activities for two reasons. Firstly a small baby sitting still feels the cold considerably more than a fully grown adult toiling uphill carrying a small baby. Secondly if the adult stumbles and slips a baby is considerably more prone to damage than the adult (consider using trekking poles to prevent stumbles). Ski-ing with a baby carrier is definitely a no-no and is banned in most ski resorts.
A child can be carried in a backpack until they are prepared to walk, though in the case of some lazier individuals you may not be prepared to wait that long - they can get very heavy!