And at the same time, the first 4 seasons of Lost take place over the span of a few months. Season 4 is literally about one week long.
But that's an example where the shows cover considerably less than real time. What we're talking about here would be a situation where the series covers considerably more than one year per season. There are a few cases that fit, though.
Alias had a 2-year time skip at the end of season 2.
24, by necessity, skipped years between seasons -- and of course it's the ultimate example of a season taking less than a year, since each season is only one day long. (Although not the only example. The short-lived 2006 series
Day Break was about a protagonist who repeated the same day over and over,
Groundhog Day-style, so technically the season covered only a day or so.)
Battlestar Galactica's first two seasons spanned only 9 months, then the finale jumped forward a year or more.
Merlin had several time skips of 1-2 years between seasons. And there's the extreme case of
Fringe, whose final season was set about 25 years after the previous season (although only a few years after from the characters' subjective point of view, since they were in stasis for most of it).
Eh? Heroes (Hayden Panettiere) and Friday Night Lights (Aimee Teegarden) did just fine with under-18s in major roles.
You're talking about actors who started out at 16 or 17. From what I can find, actors that age are allowed to work six hours a day, but actors age 9-15 -- which is what you'd need if Bruce is only 12 -- can only work five hours a day.
Also, both of those were ensemble-cast series, so no actor would've had that large a percentage of the total runtime. If what's been reported recently about
Gotham is true, then it would be at least as much Bruce's story as Gordon's, so they'd want an actor who can handle a central role. I wouldn't
want them to work a child actor that hard, even if they could. The things I'm hearing about the show would simply make more sense if they cast an older actor, for reasons of availability, experience, and in-story chronology. As suggested above, even if they start out with a younger Bruce, a major time skip at some point -- probably fairly early in the series -- would seem to make sense.