\startbuffer
\startTEXpage[offset=10pt]
    $a_{\actuarial{n}}$
\stopTEXpage
\stopbuffer

\starttext
                                \getbuffer
    \setupbodyfont[palatino]    \getbuffer
    \setupbodyfont[cambria]     \getbuffer
    \setupbodyfont[xits]        \getbuffer
\stoptext
