For the lower level character to get the bonus when grouped with a higher level character on a linked account, the mob needs to not be gray level to the high level.
How many levels this means varies by level, but a maximum of 8 levels should be enough i think, so when your toons hit lvl 63 or so they could group with your 71 and get the bonus, except the bonus ends at lvl 60.

To group your 71 with them and them to still get the bonus, the 71 would have to be on an account that is not linked to any of the low lvl ones.