Why do they need an "Easi Card" in the first place? If all under-21's are able to travel free, all they need is ID! What about out-of-towners who want to travel for free when they visit?
(Granted it would be easier to get through the fare gates in the city with a card.)The idea has merit. If you get younger people in a culture of travelling on public transport, they are more likely to use public transport as they get older. This would in turn increase patronage (and therefore revenue) in the future.
The big problem is that it would increase patronage now - when services do not have the capacity. We're still playing catch-up and until the issue of capacity is resolved, such a scheme will only harm the general view of public transport.