The Howard Hawks one, not the Marx Brothers one. Though, while I've never seen the latter, I have a sneaking suspicion that the two are pretty similar on a tonal level (which stands to reason, given the title).
Monkey Business is something of a throwback to the screwball comedies of a slightly earlier era. It doesn't necessarily reach the same delirious heights as a few other Cary Grant movies, but, between the fountain of youth premise, the two lead performances (especially Ginger Rogers, who's really going all out here), and Hawks's snappy direction, it mostly compensates with similar amounts of buoyancy and energy.
It's also got some of that characteristic Old Hollywood charm that you can't help but smile at, from the the director saying "Not yet, Cary" when Grant opens the front door prematurely (I'm not always big on fourth wall jokes, but that one was cute and creative enough to tickle me), to the adorable and surprisingly elaborate chimp antics, to seeing our dignified actors behave like juveniles. Sure, the whole thing's pretty rudimentary, and not exactly clever or consistent, but it's also affable, amusing, and animated enough that I can agree with its "minor classic" status.
Grade: A-