biking laws in Ontario?

In Ontario a bike is considered a vehicle and cannot be ridden on any sidewalk. 

http://www.mto.gov.on.ca/english/pubs/cycling-guide/section5.0.shtml

However, cities may have by-laws that allow certain size bikes to ride on sidewalks.  Normally the bylaw will state that a bike with 24" or smaller tires can ride on the sidewalk but it depends on the city.

Why do you want to ride on the sidewalk? 

I am extrapolating from your question, but I've ridden since I was much younger and stopped riding on sidewalks when I was six. No judgements, but where is this question coming from?

It is illegal and people are ticketed, even in the TO watershed.

That was an interesting extrapolation.

My GF has three kids, 18, 14+14, and goes riding with them. Wondered if it was legal for her to ride on the sidewalks with the younger ones, and if it's illegal for the 18 yr old.

Wanted to get my facts straight, so I asked.

Call your city, and ask to talk to by law enforcemnt to get the rules you have to obey in your particular region.  They are very helpful and usually quite happy to explain.  Different municipalities have different rules.

That being said, enforcement is very different from having the actual law on the books itself.  If your not aggrevating anyone else, and it isn't a busy street, etc. chances are by-law/police have better things to do and they won't bother you anyways.  This type of infraction is largely complaint driven.  If your plouging down a busy sidewalk with little regard for pedestrians, etc.   get ready for every ticket under the sun for not having a bell, helmut, etc.!