Under UK legislation the earliest you can collect a puppy from a breeder is 8 weeks old. This is the minimum, not a recommendation. Barely weaned and with hardly any bite inhibition, such a young puppy require a LOT of attention. A third vaccine is required at 10 weeks old then Puppy can enter the garden to commence toilet training. Until then Puppy must be kept in the kitchen and not allowed into the garden or exposed to dirt. This is because the vaccine is not fully effective until 14 days after the last booster which typically means that potty training commences aged 12 weeks old.
Until this age then you are at risk of exposing Puppy to Parvo and other potentially life-threatening diseases commonly found in the environment around us, particularly near foxes or rats.
At 12 weeks old, if we are training puppy we then start lead and clicker training. If you are simply looking for a puppy ready for the garden but without the training then 12 weeks old is the best age to collect.
However, if you prefer to wait a few weeks and walk to the car with a puppy on a lead and more capable of comprehending the "hold" for a wee, then 16-17 weeks old is the answer. Still, a baby puppy, but yet under basic control. Adoption changes from handing over a baby in arms to a demonstration on walking to the left and getting the children to play clicker games. Some of our families adopting at 4 months say that they are on the school run the same day with compliments on behaviour that others wish their 8-month-old puppy was so well mannered!
So have a good look at your lives, your calendar, your budget and you'll be able to answer the question - the best age is the one which is right for you