60 is a command that simply tells you how much sixty times a character's age is, as well as showing when it will have been that amount of time since the last time that character was online.
It's useful because characters will idle out and disappear after either two weeks, or 60 times their age--whichever is greater. So, for characters over a few hours old, it tells you when a character will idle out.
Forms of syntax available for the command "60": 60 60 alts by date 60 alts 60 <player(s)>
The first syntax will show 60 times your own age, and how long that will be.
The second and third syntaxes will show 60 times the ages of all your registered alts, with "by date" causing them to be sorted by soonest to most distant idling-out time.
The last syntax shows the information for another player or players you list.