There is something wrong about immigrants telling us that we should change our electoral system, which has arguably served us well for decades (I don't count the period when aboriginals and women didn't have the right to vote).
You could argue that the 18 years of age cut off is a tad high, or low, depending on the teenagers you know. But few, I think. would argue that fresh off the boat immigrant non-citizens should have the right to vote. They haven't adapted. Because even if you only moved a few hundred kilometers across the New York-Ontario border, you are still a foreigner.
You can receive Canadian citizenship rather quickly (3 years I believe). That is a good thing, as not everybody is into democracy, and not having the right to vote doesn't exactly help us sell the concept to new residents. In 3 years or so, you can learn the basics, and start to be integrated into Canadian society.
More importantly, new citizens( and teenagers) have little impact, because they are spread across many different ridings. With the MMP system, fringe groups (religious, age, ethnic origin, heck, even Americans) could each have their own MP. Is that a good thing? Not here in New Brunswick. No thanks.