There is one offense in the NFL that supports WR1 numbers from the slot. His age won't change that.
I'm not sure how you're defining slot. If it means the position they play or where they line up on the field, that statement is completely wrong. If it means their average depth of target, it does make it harder, but I still think it's a false statement. I think there's a conception that low aDot means low RZ utilization, which caps fantasy stats, especially in non-PPR. However that's not the case with Cobb, who had very good RZ utilization numbers. Even if you create an artificial aDot cutoff to create a new definition of "slot WR," you still have to include Harvin and Welker, and you can still point to subseason production from Collie, Cobb, and even Amendola to point to other possibilities. Making some artificial aDot cutoff seems pretty sketchy though. Do you not want to call USC Steve Smith a slot receiver (WR1 in 2009)? I'm really not sure what makes a slot receiver a slot receiver if it's not lining up in the slot, which makes your statement pretty bad. If by "supports" you mean systematically has produced for 6 years, then that's true. But it's not the naughts anymore. Brandon Stokely's one good year isn't an anomaly anymore. Slot WRs are putting up good years.