Knives which open automatically (via spring or centrifugal force, i.e. switch blades or butterfly knives) are prohibited weapons and may not be possessed in Canada. Carrying a concealed weapon is also illegal in Canada. This includes belt knives, sword canes, fist knives, etc. and keeping a normal knife in a pocket.
Otherwise, knives are perfectly legal. Lots of people own knives... kitchen knives, hunting knives, fishing knives, divers, electricians, construction workers, cooks, etc. Any one can go to a Canadian tire and buy a dozen steak knives, a pocket knife, or a hunting knife. Individual stores might have policies on selling to extremely young children.
By law you can't carry a conceal weapon. This would generally mean a pocket, concealed belt scabbard, etc. It needs to either be visible or stored (say a backpack, container, case, etc.) However, some common sense applies. If you are hiking or camping, police aren't going to think twice about a small pocket knife actually being in your pocket. If they find one while you are on a playground in a city... they are less likely to let it slide.
It is also illegal to bring a weapon to a public gathering. This includes any event, school, public office, etc. so you need to be careful about that also.
There is no minimum age, but again common sense applies. At some age, parents are going to get charged or investigated for child endangerment by permitted extremely young children to have a pocket knife. If you are a 12-year old working on a farm in a rural community, they likely wouldn't think twice about it. An 8-year old in downtown Toronto... is probably dangerous.