The nature of car use will change too. With automation. That said, transit use is only starting to approach European levels of service. We're not yet close to the reliability you see in other major cities around the world. You'll see changes, not just with young people, but with residents all over this city on transit use. The determinant isn't age. It's access to great transit service. Look at the difference in transit use between Scarborough and Pickering or Markham next door. Or Etobicoke and Mississauga or Brampton.
Markham's problem is the result of lack of fare integration and a very long boundary.
Lower income in Etobicoke and Scarborough and those residents not having to pay double fare to use TTC subway can't be ignored either.
Plus, Scarborough is denser, more urban than Markham. And places further away from downtown will probably have less transit ridership anyways.
Brampton has limited connections to TTC, but Mississauga's transit ridership and service levels near the Toronto border aren't actually very different from Etobicoke:
Mode of transportation of work, public transit, 2011
Etobicoke North 27.6%
Etobicoke Centre 24.4%
Mississauga East-Cooksville 19.9%
Mississauga Centre 17.0%
Mississauga-Malton 16.5%
Notice how people of Etobicoke North are more like to use transit than people of Etobicoke Centre. Do you think Etobicoke North has better transit service than Etobicoke Centre?
Service levels follow ridership, not the other way around. TTC, Mississauga Transit, Brampton put extra buses to deal with overcrowding. They can't afford to ignore overcrowding and have buses running empty and hope the riders come. YRT tried that with VIVA but eventually they had to cancel routes because no one using them.
I think transit ridership trends are very long term. More and more young people using transit now. Old habits die hard, so likely those young people will continue to use transit when they get older, and their children will too because they were influenced by their parents. It's a change in culture.