So long as prints are washed properly, they shouldn't "age" perceptibly; you shouldn't expect to see substantial deterioration of a FB print just from the passage of a few decades. The real problem is that the silver in a print, if not protected, is very vulnerable to attack by atmospheric pollutants. If you happen to be in a relatively clean environment you may never see a problem. But you can't be sure of how your prints will be stored and displayed once they leave your hands.
Unfortunately, research findings in recent years indicate that, contrary to long-standing dogma, light toning with selenium offers little protection. Some are convinced that this is attributable to an undocumented change in the Kodak Rapid Selenium Toner formula over the years - if I recall correctly, it may actually have been a sulfide contaminant that was providing the protection - but for most of us the details don't matter. The bottom line is that the product available to us today doesn't do what the conventional wisdom says it does.
Toning to completion in selenium does protect, but causes substantial change in the appearance of the picture; I've done experiments on toning to completion with selenium using several different FB papers and didn't like the results at all. Similarly, sulfide toning, while highly protective, radically changes the character of the print.
These days I don't hassle with toners at all - I just dunk my prints in Sistan and leave it at that. Yes, there's controversy over just how protective Sistan is over the long run - there are no guarantees. But Sistan is cheap, easy to use, and doesn't change the way the prints look. I decided that it's a reasonable precaution to take - beyond that, I'd rather spend my time, money and energy making more prints rather than hassling with toners that are a nuisance to use and produce effects I don't like.