Should I replace my breaker box due to its age?
Don't listen to the guy at the store telling you to toss out a perfectly serviceable QO box.  Parts for the QO line are available through practically every supply house in North America, and most of the big-box stores too -- it's quite common to this day in both residential and light commercial work, and as far as I know, hasn't been altered in a backwards-incompatible way since its introduction in 1955.

As a general rule -- if the panelboard is of a type still in use today (i.e. QO, Homeline, CH, BR, QP, or THQL), has the ampacity and physical space (incl. tandem/double-stuff slots), and is in good condition inside and out (no hot spots, arcs, or burnt insulation), it's a keeper.  It's those pesky FPE, Zinsco, and to a lesser extent Pushmatic panels that need to be sent to the smelter....