The age range I see recommended most is between 8 and 12 weeks.
What problems are one likely to encounter with an older puppy?
How quickly are these problems likely to show themselves after the recommended range? That is, I’m guessing the puppy’s behavior doesn’t undergo a dramatic transition at the stroke of midnight on the last day of the 12th week… how far would it be wise to push things, age-wise?
Socialization (exposure to the widest variety possible of people, dogs, and situations) at the proper age is key for a well-adjusted dog. Provided the breeder of the puppy has done this for the litter (as most well-educated and reputable breeders will) there’s much less to worry about. But the ‘window’ during which much of a dog’s lifelong ability to approach life with confidence develops closes between 16 and 20 weeks depending on the breed and individual. If you buy a puppy that’s been sitting in a cage in a petstore from weeks 8 to 15, expect some difficulties, maybe forever.
Dogs that haven’t been properly socialized aren’t all basket-cases or agressive, of course. Most dogs probably don’t get nearly what they should at the right age. But we all want a dog who’s happy and secure, and doesn’t have serious behavior problems we must try to solve or clean up after. So respect this rule for the best outcome.
At least 8 weeks old because earlier than that and you’ve got a pup that isn’t completely weaned off of mom’s milk yet. Vaccinations start at 9 weeks.
In my opinion, I see a lot more people who are having behavior problems with their new pup than with an older dog. Mostly, the new puppies have issues with housebreaking. Older pups seem to have figured out the whole Designated Pee Area idea and just need their new environment explained. Older pups are also in a better frame of mind to learn other tricks, which makes them more interactive and fun (not that young pups can’t be taught tricks, just that their attention spans are shorter and trickier to work with). There are also some disease that are more common in young animals.
Other advantages of adopting a pet about 6 months old or older from a shelter:
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[li]You know their final body size[/li][li]they usually come spayed/neutered[/li][li]they need fewer vaccines (both in total number of diseases protected against and total times a particular vaccine is given)[/li][/ol]
All but the most severe behavioral issues can be worked on in adult animals. The adage that you “can’t teach an old dog new tricks” is a total lie. Also, just because you socialize a pup doesn’t mean it won’t have quirks. Some dogs just don’t like cats or little kids or biker dudes or what have you.
The main thing is establishing clear, reasonable rules that you stick to. Every animal requires consistency and breaking the consistency will lead to problems.
Or, you could get a cat and not have to worry about half of this bullshit. 
In Soviet Russia, cat trains you! Actually, everywhere in the freaking world, cat trains you.
Tho I do not recall too many specifics, I remember reading that 8-9 weeks is just about optimum. Before 8 weeks, there is too much development and socialization going on. But too much after that time, the individual puppies begin to manifest their personalities in ways that might be detrimental. The more assertive ones might bully the more passive ones, for example, in a manner that might foster undesireable long-term behaviors.
Also, from 8-16 weeks or so, the pup is extremely plastic and receptive to influence in terms of what its lifelong personality will be like. So you want to have the pup during that period, to encourage it to adopt traits/behaviors that you prefer in a dog.
I’ll see if I can dig up any titles. But I remember reading more than one book (which appeared reputable to me) discussing the specific timeline of development during the first weeks/months.
Agree that most of the problems I have seen are from separating the puppy from mommy* too early*, before 8 weeks, and not from later separation.
Philosophically, you want the puppy’s primary interaction to be with the mother and litter before 8 weeks, and thereafter increasingly with (consistent, kindly, committed) humans, to transition the puppy into living with humans. The human(s) working with the puppy could be the new family, a foster family, or even that rara avis, a truly committed breeder; it doesn’t really matter who, as long as the people in question are knowledgeable, patient, and putting in the time.
Not my cats. It’s definitely possible to outsmart a critter with a brain the size of a macadamia nut, and figure out how to train them to respond to commands.
Before 8 weeks the puppy should be with it’s mother (milk, etc.)
No puppy will be socialized at 8 weeks, but it’s not that hard to do.
You can retrain a dog at any age; don’t let anyone tell you you can’t. I’ve done half a dozen times, and it’s no more difficult than training a puppy from scratch. You might get an occasional bad habit which is hard to shake with an older dog, but anything’s doable. As long as it’s not aggressive towards humans - that’s the hard one to deal with. Again, it often can be done, but it’s a big challenge. Fortunately, it’s also very rare - someone had to have really screwed that dog up first, because that’s not their natural instinct.
I think part of the reason people don’t get puppies a bit older is that they want to enjoy how cute they are when they’re small for as long as possible, rather than due to drawbacks in getting a slightly older puppy
You’re probably right. But, IMHO, critters are way cuter when they are interactive and young pups don’t have the attention span for much interactivity. Gimme a teenager to young adult critter any day.
Scratch that. Gimme a middle-aged dog where someone else has taught them a bunch of stuff already, because I’m lazy. 
Haha, yes. Puppies are just small, spazzy dogs with no manners. I can’t see myself ever raising another one. Give me a full-grown shelter/rescue dog any day.