as a vintage perfume aficionado myself, having purchased several bottles over the years, the answer to your question is "it depends" - on how *each* bottle has been cared for/stored during that time....

in theory, a bottle of perfume that is still sealed *can* be good "indefinitely" - in fact, some experts say that high quality perfume, like fine wine, can actually improve with age...

however, oftentimes, this is totally not the case....as a sealed bottle of perfume that has *ever* been exposed to any extended periods of heat or sunlight over the years *will* go bad...

unfortunately, it's usually impossible to tell whether a vintage bottle of perfume is still good just by looking at it...especially in the case of heat exposure (as there will likely be no change in the color of the perfume or the look of the label, which can sometimes be a tell-tale sign of past sunlight exposure)....to sum it up: there's just no way to tell without actually opening the bottle.

also, even though the *seller* may be legit, they may have purchased the collection from another party - and, that party may very well have acquired those bottles from *several* sources.....therefore, some bottles within the collection may still be good and some may have gone bad..again, it's really a toss-up, without knowing how each bottle was stored/cared for over the years.

if i were you, i'd decide whether to buy vintage vs. new based on what you think your niece's intent would be for the collection - for instance, if she'd want to keep them on display, then, by all means, get the vintage collection! (i know that's what i'd want if i were a pre-teen or teen!)...however, if she wants to actually *use* the perfumes, your much safer bet is purchasing fresh - and they'll most likely be less expensive, to boot.