To give you an idea, here are some things you can do at certain ages here in the UK. Try to decide for yourself where the age for buying a dog should be.
At age 10 you can:
Be found guilty of a crime.
Be finger printed, searched and interviewed by the police
Be Locked up for killing someone.
At age 13 you can:
Work part time in certain jobs only
Be found guilty of rape and other sexual offences
At age 14 you can:
Be found guilty of any crime
You can ride a horse on the road without wearing a riding hat
Drive a tractor on farmland
At age 15 you can:
Be sent to a young offenders institute or prison.
At age 16 you can do alot more things but only with the legal consent of your parents eg:
Make a parachute jump
Enter a civil partnership.
At age 17 you can:
Leave home with your parents consent
Drive a car
At age 18, you think you're all grown up?
Well you can now buy fireworks, cigarettes and alcohol
You can own your own home
You can serve on a jury and you can borrow money
You can vote and also be tattoed
But you can't drive a lorry, bus or train, nor can you
demand the National minimum wage.
So if you as a breeder of dogs were to be asked to sell a puppy to a young person, would you know the legal age?