If crocodiles are k-selected, why do they supposedly "not age," as in avoid senescence?
Comments

You seem to use the term evolutionary advantageous vs disadvantageous for the species rather than for individuals. It is a very common misconception from the general public. You might want to follow an introductory course to evolutionary biology such as Understanding Evolution (UC Berkeley) for example.


  younger crocodiles with more slightly more fit genes


"fit genes" does not mean much. Again an introductory course to evolutionary biology should help you there.

Answer

You'd right though that absence of senescence causes longer generation time and therefore lower response to selection pressure (such as those caused by an environmental change). But populations don't evolve in order to maximize the probability of the survival of this population (related to first comment above).

Those that live longer, leave more offspring and therefore long-lived individuals are expected to dominate the population in the long run. The evolution of senescence is not exactly that easy though otherwise no species would show any sign of senescence (see Why does evolution not make our life longer?).