Not sure how old my puppy is?

Basically i bought a puppy and the lady told me he was 8 weeks. few days later i needed to find out his date of birth and the date she gave me indicated that he was actually 6 weeks old when i bought him.well now my puppy is 18 weeks old and im not really sure how old he is to be honest and there are 2 reasons.the first reason being that i read online that dogs lift there legs up to piss when there about 6 months and he started doing that when he was 16 weeks.the second reason is he got an erection yesterday and he is only 18 weeks im not sure if puppies get them that early so i need help!

ASK THE VET!I gather you have taken the puppy to vet for vaccination needle ?This woman should be able to provide you with papers in regards to vaccination. Meaning if he is 8 weeks old, he should have had at least 1 vaccination needle.It is extremely IMPORTANT that puppies have vaccination needles for their own health and safety.Refer below:At 6-8 weeks of age puppies should receive their first vaccination; this is temporary and needs to be followed up with another one at 12 weeks. After the 12 week vaccination you can then take your puppy out in public areas.What do you need to vaccinate against? •Parvovirus – a highly contagious viral gastroenteritis. Depression, loss of appetite, severe vomiting and diarrhoea containing blood are some of the symptoms. Death can occur very quickly. •Distemper – a highly contagious disease producing symptoms such as conjunctivitis, nasal discharge, convulsive seizures and spinal cord damage. Treatment is often ineffective. •Hepatitis – in puppies can cause sudden death, whilst adult dogs can experience, weakness, fever, diarrhoea, loss of appetite and bleeding. •Canine Cough – a complex disease caused by bacterium and a virus. Affected dogs will have a hacking cough persisting for weeks. In puppies and old dogs the disease can be devastating.PS: I hope you are also worming him and providing flea treatments..otherwise, you are not a good dog owner if you neglect your pet.

You bought from a BYB :\ You should really pay attention to where you are buying from, and who.