My personal opinion is that the whole young sidekick wouldn't work from a newly created character nowadays. The idea of a (supposedly) sane and mature adult putting a child in danger doesn't generally sit well with a lot of people. Having said that, it can work so incredibly well. Look at Kick Ass, while clearly a parody of Robin the character of Hit Girl is awesome and can fend for herself better than a lot of the other heroes out there. (If they introduce Robin in these new films, totally the approach I'd go for.)

In terms of Legacies, Arrow does it well enough with Speedy (to the point I could see Thea taking over the show if Stephen Amell wants to go) and it's looking like the Flash is gonna head in that direction eventually with Wally West. I would say though that it only works because they spend a lot of time on these characters in a way that most films and even comics can't. 

[@Morden Man] I'd rather not see John Blake, to me that was a bad example of the movie wanting to have its cake and eat it too but that's probably a discussion for another time :P

I do think similar-age partnerships are the way it'll go, in the movies at least.