I don't agree, unless if 'aged well' means 'no challenge and debilitating story' like most JRPGS since.
II still has one of the best scenario I've ever seen in a game. And it's challenging, but fun (you -really- fear some encounters in dungeons).
By hasn't aged well I mean, the level cap of 30, with required grinding to proceed with the story. The menu and interface being unwieldy in this day and age, and the game pulling some seriously sadistic moves at various points, without warning. For all the good it did, it's still a 1980's RPG, and it comes with all those trappings, both good and bad.
As for SMT as a whole. No, I advocate playing the whole series, especially if one's interested in moving onto the modern games. SMT IV works on more levels when you know the plot to I and II, and can pick out the musical nods, and callbacks.