besides the fact that people hardly EVER read the posts above them and barge straight from the OP to their own $0.02, this post is FILLED with wrongness (and a little rightness).
as mentioned before...
...you DO NOT GAIN CHARACTER XP after 50
...you DO, MOST DEFINITELY gain legacy xp from characters at 50.
and that's my $0.02...paid in Canadian coins!
I would like to add:
...you only need to ever roll one char to eventually make it to Legacy 50 and to gain access to all Legacy features and perks (the Legacy pet unlock in 1.3 may be the first exception to that rule, for it you will need one Rep and one Emp lvl50 char)
...rested xp bonus (the green on your xp bar) does not apply to lvl50s (you may now speculate if the rested bonus does boost Legacy xp progression while leveling up Alts or if its still faster with your 50)
...at lvl 50 your character xp will show 0, once the lvl cap is raised everyone will have to xp grind through all of lvl50 to reach lvl51 and beyond.
However, just lvl up to 50 and relax. The game does not stop at lvl50 but just begins. It also helps to read all the Information ingame that is available on your Legacy window.