Yes. They are interchangeable according David K. Jordan, in his book titled Being Colloquial in Esperanto. (I don't have the page number for this reference).

In the Tekstaro, plej mal- occurs 800 times. Whereas malplej occurs 67 times.

In Google, a search for "la plej maljuna" (with quotes) results in 50,400 hits. Whereas, "la malplej juna" (with quotes) results in 73 hits.

Based on this info, it is more common to use la plej maljuna instead of the other syntax.