Personally I prefer 5th and 6th years, though as [@unicorgi] said, that takes the fun out of the "getting to know you" phase of a HP RP. With that being said, making it with 1st years wouldn't be very exciting unless the OP is willing to make a very active plot as opposed to a sandbox.

I'd think a compromise would work; 3rd/4th years would be [i]just[/i] unfamiliar enough with people outside their circle of friends/houses to have first meetings, while being old and experienced enough to add some variety to player choices. We could work with that if people are undecided.