Technically speaking, all of them will eventually yellow, but as far as I am aware, later model SNES do not have the problem of premature yellowing.

Yellowing is caused by an oxidation reaction of the plastic with the air, but a certain fire retardant additive present in certain earlier batches of SNES plastic causes the plastic to oxidize much faster than normal. This process can also be sped up by overexposure to Ultra Violet radiation, though this will usually result in a more uneven yellowing pattern.

I wouldn't worry too much about it. If it does get yellow, then there is always that Retrobright stuff.